Apple phones can send texts and communicate with any other phone on any other network and vice versa. They just don't use iMessage to do so. I can't DM my discord friends from my work Slack, for example.
As for why it's anti-trust, it's right there in the WSJ article. Apple executives are explicit in the lock-in being useful for their market power.
Also it may have been years ago but I'll never forget the way Apple originally captured phone numbers into iMessage. Once a phone number was registered in to iMessage no Apple system would ever send SMS to it again. That created a major problem if the owner of the number had stopped using Apple phones. They were sued over this lock-in and finally implemented a way to release your number from iMessage jail. It's still kind of awkward though.