Not sure I understand what you mean. X had the number, so the fact is reflected in some searchable dataset. Then abandoned it. Maybe there was no WhA registered, or it expired after 120 days. Now Y gets the number from a mobile operator pool, and registers WhA.

>their logic of making the phone number the account

I see nothing wrong with it. It's clear and long-established offer: number=account, and WhA definitely neither the first, nor the only one who does it. Of course, it has its disadvantages, but it's ok for the majority of users who don't want to show id, or fill a questionnaire to start texting.