It would simply be bad writing to set up a mysterious and malevolent figure like Hat and Cloak and then reveal him as one of the story's established villains.

Unless the reveal involved learning about the Voldemort-Quirrell symbiosis, or Voldemort-Hat-and-Cloak outsmarting Voldemort-Quirrell, or any of a dozen other dramatic reveals.

A point in favor of Hat and Cloak being Grindelwald: the playing card he chose to represent Dumbledore was the King of Hearts. ♥

At first I wanted to say "reading too deeply", but you have a point: the choice of card was not a throwaway line, it was intended to be mysterious, so it should have some depth worth plumbing.