A definite problem from a company's perspective when their model is ads and gold/donations, both of which require users to be logged in. This directly affects the number of users willing to participate in discussions - both in big subreddits and niche ones - and it risks some active users becoming dormant because they dont see enough reaction/response to what they have posted.
Stack Overflow is not the best comparison. In some cases yes, they are very similar, but mostly the reason for users to land on stack overflow is very different (solving a problem while working) than for users to land on a subreddit/post (looking for more info and satisfying curiosity via Google search - not always solving a problem)
I think it is similar in this sense: those users are in a funnel as long as they visit regularly. Not everybody who walks into a store buys something, but if the store is busy that’s a good sign.
There is a distinct possibility that those users arent visiting regularly. They visit via Google search, explore a bit, don't create an account. Next time they come in, it's again via Google search. Reddit may or may not be able to track them.
And again, you cant see that they are even visiting a page. All the signals - upvotes, comments, etc. are only available when logged in. (except for video views)
Stack Overflow is not the best comparison. In some cases yes, they are very similar, but mostly the reason for users to land on stack overflow is very different (solving a problem while working) than for users to land on a subreddit/post (looking for more info and satisfying curiosity via Google search - not always solving a problem)