Quirrell == Voldemort. This does not imply that Voldemort == Riddle. Dumbledore at least thinks that Voldemort was Tom Riddle, but consider the possibility that Dumbledore is wrong about this. Remember Voldemort really doesn’t look very human at all, and could be almost anyone. Maybe Riddle was misdirection.
Quirrell == Voldemort. This does not imply that Voldemort == Riddle. Dumbledore at least thinks that Voldemort was Tom Riddle, but consider the possibility that Dumbledore is wrong about this. Remember Voldemort really doesn’t look very human at all, and could be almost anyone. Maybe Riddle was misdirection.