> Not if they integrate the messaging infrastructures as described in the article...

WhatsApp is a phone app. Even if Facebook heavily integrates the messaging infrastructures, the problem an acquirer has is porting the existing users over to a new messaging infrastructure. Signal-based infrastructure is (relatively) turn key, most of the software is already developed, deployed, and tested. After you have that, the main thing you have to do is push a new version of the app out to all the different app stores that uses your new infrastructure. Bam, you're done.

I am simplifying certain things (there'd definitely be a somewhat complex transition period where your new app would have to support both infrastructures), but my main point is that this integration is not as big of a barrier to re-separation as it may seem.