So you're right, they didn't extend the Gemini protocol, they created an entirely new protocol which many clients, servers, and libraries now implement because the functionality was desirable.
Wonder what they'll call the protocol that implements DELETE... maybe Deorbit?
Your use of "they" here is misleading. Many, if not most, people in the Gemini community see no need for Titan. The existence of a new protocol doesn’t imply that Gemini is somehow lacking.
A Gemini client that doesn’t support Titan is still a fully-featured Gemini client.
Additionally, your description of Spartan is simply incorrect. There are several significant differences between it and Gemini - the most obvious being that Spartan doesn’t use TLS at all!
You're right, I mischaracterized Spartan. Although it is very similar to Gemini, it's not compatible because the author decided even 2-digit response codes were foolish luxury, and as you mentioned there's the TLS thing. Of course, we still call it HTTP even when it transits a TLS connection, but that's not The Gemini Way so...
Then it's not Gemini. Interestingly this is why it was decided for Gemini not to have a protocol version. To prevent extension.