What we're not seeing is granular data on their userbase, not that we ever will, but I'd venture a guess that it's much more damning than that 15% represents.

To illustrate my point, take my experience with the Reddit API changes. This was the absolute last straw for that shithole for me.

I still have an account on Reddit. I still dip into a couple of very niche subs to check on them and to provide info to people where I can, but almost exclusively through DMs. Sometimes articles redirect me to a Reddit source. Fairly commonly, search results for things like tech fixes direct me to Reddit.

I'm still a Reddit user (🤢🤮)
I would still be represented in their monthly user stats.
Even if I deleted my account, I would still visit Reddit at least once a month (this is even more applicable to how closely connected the news media and Twitter are.)

Do I comment on Reddit anymore? Nah.
Do I upvote or downvote things on Reddit? Very rarely.
Do I spend any significant amount of time browsing Reddit these days? Nope.
Have I switched from being an active contributor on Reddit, which is probably like 10% of their total active userbase, to being almost entirely passive? Yep.

Idk (because I don't care about Twitter beyond enjoying watching it implode and the anticipation of this potentially taking Elon with it) but I have a hunch that they've had a huge decline in their active contributors and especially those who made Twitter a valuable social media site (in the sense that you're always going to get trolls, bots, marketers and those really vapid chain response tweets as a form of low-value content but there's a certain amount of active contributors who create something of value which attracts other users - and those are the really important users. If that cohort of people shift from high-value contributions to passive consumption or occasionally dipping in only to answer DMs or because of a link redirecting them to Twitter then the site is moribund.)