At first, I fully agreed with you. Then I saw another comment using "an experiment was conducted" as an typical example of passive voice, and I find that to be far more passive:
"...was conducted" suggests no information about the conductor. It could be the author, and in the specific context of an academic paper it implies the author, but it could just as well be anyone else on Earth especially outside of this context.
"Surprisingly" (or "the results were a surprise," etc.), on the other hand, doesn't leave anything to the imagination. I can't think of a way this word could potentially mean that another party but the author was surprised.
Passive voice could hide the party responsible for surprising someone (active "Alice surprised Bob" becomes passive "Bob was surprised"), but in the original scenario, the party causing the surprise (the subject, which could be hidden by passive voice), is the experiment. The surprised party is the direct object, not the subject... passive voice is not a term to describe hiding the direct object; that's transitive versus intransitive.
With all due respect, if you're not familiar with the concept of voice in linguistics (which has a particular technical definition, and makes no sense to apply to a purely adverbial phrase like "surprisingly" or "to our surprise"), maybe you should read up on the concept before trying to determine the difference between active vs. passive.
The term distinguishes between sentences where the semantic agent is the syntactic subject, and those where it is the object.
For example: "Bob threw a ball at Suzie" (active, Bob is the syntactic subject) vs. "A ball was thrown at Suzie by Bob" (passive, Bob is the object of a preposition and "a ball" is the subject).
I edited the tail end of my previous comment to touch on this, which is to say that omitting the direct object typically involves using intransitive instead of transitive. For example, the intransitive "today I ate" instead of the transitive "today I ate pie" leaves you guessing what I ate. The verb "surprise" does seem to have an intransitive syntax per https://www.merriam-webster.com/dictionary/surprise which gives a similar sentiment as "surprisingly."
I'm not sure I agree that a distinction needs to be made between the two, certainly not on lines of agency. The "our" in "to our surprise" may or may not correspond to an agent, patient, etc., in the modified clause.
To an agent: "To our surprise, we found them. To our surprise, group A was found by us first."
To a patient: "To our surprise, they found us. To our surprise, we were found by group A."
To neither: "To our surprise, group A found group B. To our surprise, group B was found by group A."
I suppose "to our surprise" is explicit about whose expectations weren't met in a way that "surprisingly" is not. But in a first-person narrative, I imagine most readers would understand "surprisingly" to mean "to my/our surprise."
This suggestion is precisely comparable to the passive voice.
There is an intended agent experiencing surprise, and we might under some circumstances agree that we know who the agent is (just as we expect that "the experiment was conducted" by the author as researcher).
But, technically, the agent is obscured and not written, so we can't be completely certain about the writer's intent. Maybe Rosalind Franklin conducted the experiment: we are only led to infer that the author was responsible.
Personally, I generally understand "Surprisingly," to mean "An attentive reader should now be surprised that...".
There's an expectation of general surprise relative to an earlier claim in the text; the writer assumes the reader will be surprised (whether or not the writer was truthfully surprised). I find myself annoyed by this style, whenever I am unsurprised.
Comparable to one use case of the passive voice, maybe.
What do you mean by "intended agent"? "Agent" has a common definition in linguistics, and an agent is only an agent in the context of a verb, not an overarching narrative. The same referent can be an agent in one sentence but not the next: "We hid. To our surprise we were found by group A." "Group A" is the only agent in the second sentence.
There may be other definitions of "agent" in other contexts, but we're talking about grammatical voice here.
The passive voice is quite simple: the grammatical subject is filled by the patient or theme of a clause, not e.g. the agent. There are many use cases for the passive voice apart from obfuscation, and obfuscation is hardly a necessary result.
I might call "surprisingly" in your interpretation a weasel word in the broad sense. The inference is that the author wants the reader to take up an attitude but is not being forthright about it.
Of course some passive clauses may be weaselly in their own right, but the passive voice is not weaselly by definition.
"...was conducted" suggests no information about the conductor. It could be the author, and in the specific context of an academic paper it implies the author, but it could just as well be anyone else on Earth especially outside of this context.
"Surprisingly" (or "the results were a surprise," etc.), on the other hand, doesn't leave anything to the imagination. I can't think of a way this word could potentially mean that another party but the author was surprised.
Passive voice could hide the party responsible for surprising someone (active "Alice surprised Bob" becomes passive "Bob was surprised"), but in the original scenario, the party causing the surprise (the subject, which could be hidden by passive voice), is the experiment. The surprised party is the direct object, not the subject... passive voice is not a term to describe hiding the direct object; that's transitive versus intransitive.