The difference is that the email protocol has long been established and any new email client is built to that protocol standard. What we have here is an open protocol still being developed. The fear is that FB will force changes into that protocol and take it over. Then it will no longer be an open development protocol. By expunging FB right now before they get a firm grip on the userbase it can preemptively prevent FB from causing damage.

We are kind of in unexplored territory right now. You could compare it to google/MS taking over xmpp but it's not quite the same situation either.

But the reality is that the current fediverse doesnt need facebook to be successful. It already has the users to continue to grow. By combining user pools facebook would have the majority share with their instagram users which means they would have a controlling share of users and would leach users away from the fediverse over time until they broke away at which point fediverse would die as most users would be forced to follow in order to keep their feeds.

This way those feeds never mingle with FB and thus fb cant leech them.