When you ported over to Public mobile, did your original phone carrier call to confirm the port request? Probably not.
No they didnt. My understanding is carriers have said they have or are implementing some basic security to prevent number theft. Also, when I ported over to PM I wasnt aware that theft of numbers via porting was taking place.
I think that the Telecom companies are still in the process of doing something. Public mobile is part of Telus umbrella. Hopefully, once Telus figures something out, Public mobile will follow afterwards.
For now, keep a "strong" password for your PM self service acount.