Yeah, it's actually a welcome change that they're federating. However, the way they killed off the last federation we had with XMPP was through the EEE model -- they first acted friendly, joined our federation, then they ensured their client would be the best featured, capturing a majority of the people in their user base, and after that they defederated and the community collapsed in their favor. People on non-proprietary solutions had to switch to the proprietary one.
To avoid this, we need to defederate while we're still ahead. I'd personally draw the line at 25%, but the point is just having it significantly less than 50%. If they defederate before they reach a majority, the community will collapse in our favor, and people with proprietary accounts will be the ones forced to come over here. Worst case, we'll just exist beside each other as competitors, and in the best case we'll snuff them out.
We need to be willing to do this to them, because they absolutely will do this to us. Threads is developed by the same Meta who helped kill XMPP a decade ago. (And "helped" only because the main culprit was Google -- regardless, they're not our friends.)