You make it sound like they claim subreply is the thing that might have killed mastodon. The full post is:
Mastodon is dead. Most people have migrated to Bluesky or Threads.
Thank you for the context. It’s important and often overlooked.
...and completely wrong, as evidenced by simply googling the words "fediverse stats", which will show you no meaningful change one way or the other.
Most of their other posts on the front page also aren’t exactly charismatic (or plain childish). A good early warning sign to stay away from that place!