It's just another attack on the legal system. Trump can say what he wants, but there is no legal basis for those claims. The president has pretty much unfettered ability to grant pardons. The Constitution of the US doesn't say a whole lot about how they have to be signed, or if they have to be signed, or possibly if they even have to be written down. There is some evidence that Lincoln issued at least one pardon verbally, and that makes a whole lot of sense considering when the Constitution was written.

On the other hand, Trump said he would investigate people who had been given pardons. I doubt that violates any law written or implied. He can investigate whoever he wants to, and in whatever detail, what he can't do is charge them with a crime for what they have already been pardoned for, which he doesn't claim to be doing. If he did that, it could sort of amount to a constitutional crisis, but probably not. The court would dismiss the charges. If that was the end of it, then no crisis.

This is just more of Trump trying to silence critics by attacking anybody who speaks up. One more play from the fascist playbook.