Very true; I've forgotten whether wizards can create false memories to cover up big time gaps. If they can, it's a much smaller problem than it looks.

But we have reason to believe that Cloak-and-hat was not expecting to have to make very many attempts, that he either is usually very good at the dictionary attack or he's that Dunning-Kruger - he got so frustrated he exploded and asked a revealing question outright. And then, the writing seems to imply, he only needed one more try to crack Hermione's code.

So, this reads to me like an expert using an effective tool who happened to run into an extremely unusual girl/problem, not a only-modestly-clever-or-perhaps-even-stupid person. (Also, thinking again on my remark that it's a very Dark technique, I'm even more confident that this is not Lupus or Sirius - neither of them seems like the kind of character to pull such a Slytherin technique.)