It’s so surreal that this has to be explained in this day and age.

Or more formally: "findings of fact" are part of the purview of the judiciary, definitionally. As always wikipedia does a good job here: https://en.wikipedia.org/wiki/Trier_of_fact

> Facts are still different than opinions, that statute doesn't give him unchecked power to declare any crazy idea as fact.

I don't know about the president, but IIRC, juries have a quite wide latitude decide what facts they find ("In Anglo-American–based legal systems, a finding of fact made by the jury is not appealable unless clearly wrong to any reasonable person", https://en.wikipedia.org/wiki/Trier_of_fact).

Saying "whenever the President shall find as a fact" seems like it's giving the president the authority to determine what the "facts" are, and not putting any conditions on how he does that or subjecting them to second-guessing.

No, it's precisely the opposite. The choice of that term isn't to empower the president, it's to clarify that such a decision is a "fact" in the legal sense and thus subject to judicial review. In point of fact the page you link says explicitly that courts (and not the executive branch) are the government organ responsible for determining facts.

Facts are facts, indeed, but bact finding is determining what the facts of the situation are, and this law makes the president the one who finds the facts. This doesn't let him declare squares to be circles, but it does pretty clearly let him declare more or less arbitrarily that a trade agreement is, in fact, disadvantageous.