Why can humans solve certain "undecidable" problems?
Humans can solve some instances of that problem efficiently, but there is no reason to believe that humans can solve all instances efficiently.  Showing one instance that a human can solve efficiently does not imply that humans can solve all instances efficiently.

Undecidable means "there is no algorithm that can solve all instances and that always terminates".  There could still be an algorithm that can solve some instances, even for an undecidable problem.

So there is no contradiction.