I would like to know why he thinks it should have been stopped. At the time of that purchase I think any messaging platform could have been sold to anyone and not been stopped (imo).
Again, it's simple: he says it should have been stopped, not that it could have been stopped. Or you could read it as "in a better system, this purchase would have been seen as anticompetitive and stopped". I happen to agree with the sentiment.
Again, I ask why. It wasn't anti-competitve because the service still exists as a standalone service just with a different owner. I see what you are saying it just does not make any sense in terms of antitrust or anti-competitiveness
It's not really standalone. The privacy implications are case in point. Facebook is a vehemently anti-privacy company, and it's not going to allow WhatsApp to undermine on this point. If WhatsApp (or indeed Instagram) were truly independent then we may well see competition on this front.