To my understanding, Hexbear forked (copied and modified) an older version of Lemmy and tweaked it to some of their preferences by adding their own code so they were kind of like a cousin of Lemmy. Migrating to upstream Lemmy means they've switched back to the original Lemmy project and installed one of the newer versions without their own modifications, or at least with few enough that it appears to be running Lemmy when probed by whoever is compiling these numbers.

I think they have federation turned off on their server so you still can't follow a Hexbear community from another server, but theoretically if they changed their settings to allow federation, you'd be able to follow their communities just like any other server that runs Lemmy or is compatible with Lemmy through ActivityPub.

As for how to tell Hexbear runs Lemmy, if you scroll down to the bottom of their site it has links to Lemmy documentation, the code used specifically by Hexbear, and Join Lemmy which gives you a hint even before clicking on the code link to see for yourself.