> someone went into a T-Mobile store and somehow convinced the associate that my number was theirs.
The fact that the T-Mobile employees can get hold of your mobile phone number is disturbing and a red flag for using your phone number for sensitive stuff (such as money). You should always assume malice from unknown actors.
I don't know where OP is from, but over here(Poland) you need two forms of ID(Passport/national ID/driving licence) if you want the T-Mobile clerk to do anything for you in-store. I got quite annoyed once because I needed a new sim-card for my company phone, but despite having two forms of ID confirming that I am the company owner, they also wanted to see the incorporation papers saying so.
I think the point of the grandparent is that the T-Mobile clerk has the power to register a new SIM card for your account. That makes them an extremely weak link (easy to blackmail, corrupt, etc.).
The fact that the T-Mobile employees can get hold of your mobile phone number is disturbing and a red flag for using your phone number for sensitive stuff (such as money). You should always assume malice from unknown actors.