The article says it's to limit spam. I don't feel platforms like Lemmy (or the other platform) are particularly spammy though. On the other hand I get a lot more spam on Whatsapp, even though it's phone number bound.
Signal is pretty good in terms of limited spam, but I'm curious about the impact if they A/B test the removal and see how much spam would arise. Obviously that could only be implemented after they remove the need to add contact via phone number.