The difference is the stage at which they "advocate" for it.

People here are advocating for it now before Facebook has a chance to "embrace" us.

Facebook would only "advocate" for it after they've "embraced" us and started to "extend" ActivityPub with proprietary features that potentially caused issues with Lemmy users.

With the former, Lemmy continues on its own, growing naturally. With the latter, Lemmy users lose contact with communities they've become a part of and may be forced to move to Threads to continue interacting with their communities. That harms Lemmy's active userbase. Additionally, because of how big Threads is, it'd naturally have the largest communities, so other Lemmy users would start using them instead of communities on other instances. That means those communities would shrink and may even die off entirely. When Facebook cuts off ActivityPub support, that'll leave us with several small or abandoned communities. So we'd end up with a smaller userbase and fewer active communities.