I understand your concern, and I think the PEP addresses it. Quite bluntly, t"foo" is not a string, while f"foo" is. You'll get a typecheck error if you run a typechecker like any reasonable developer, and will get a runtime error if you ignore the type mismatch, because t"foo" even lacks a __str__() method.

One statement the PEP could put front and center in the abstract could be "t-strings are not strings".