The point is simple, the moment you have the biggest chunk of the userbase, you have more weight in establishing praxis for standards & protocols. In fact, the protocol needs to catch up with you, rather than viceversa. Google did the same with Chrome, for example. Try to start a browser today, and with all the stuff that Google forced into standards and that your browser need to comply with, you will fail. Even just forcing a pace in changes to ActivityPub can mean that a number of tools that are developed by volunteers won't be able to keep up.

Imagine Meta brings in 100m users. This is a fraction of their userbase, but it is 8x the whole fediverse. Imagine now that they make some change that doesn't comply with ActivityPub, what do you do, break the tool that is used by the 90% of the users, or adapt? And what if they push changes to ActivityPub, so that everyone needs to catch up quickly: lemmy, mastodon, pixelfed, etc. How soon before some tools with less active development will die because non-compliant? (Similarly to how some browser break with some sites)