This seems like incredibly lazy reporting. Yes, there was a flood of new signups, and not all of them became regular users. But look at the growth of actual usage and posts of the Fediverse, and it continues to go up.
I've seen a few articles with this framing lately, and it's just so lazily misleading.
https://www.wired.com/story/the-mastodon-bump-is-now-a-slump/