The Supreme Court has held that this applies to criminal law - you can't change the punishment of a crime or change rules of evidence to convict someone who was acquitted.

You can certainly pass laws to undo a merger or (equivalently) break up a company.

https://en.wikipedia.org/wiki/Calder_v._Bull

The problem's not the "ex post facto". The problem is the "Bill of Attainder". You can't pass a law that, e.g., breaks up Amazon - to do so would be to deprive them of property without trial.

But you can create a law that Amazon (and potentially others) will be in violation of, and following proper trial, they are broken up.

That makes absolutely no sense. An ex post facto law is a law that criminalizes behavior after the fact. No one is saying the mergers were criminal, nor is breaking up the companies intended to be a legal punishment.

A bunch of folks have pointed out how factually incorrect this comment is in the specifics but I'd also like to mention that amending the constitution isn't a bad thing. "Originalists" are a weird bunch since they seem to hold the original state of the document as immutable while the founders had the foresight to include remedies to amend the constitution within it... it's a living document.

Also, just to reinforce the point other folks have mentioned - that clause doesn't work like that.

Ex post facto only applies to criminal laws (Calder v. Bull). And it generally only applies to punishments for crimes. See how sex offenders had to register even if their crimes were committed before the Adam Walsh bill was passed, because registering as a sex offender is not a punishment.

Requiring a divestiture is not the same as undoing a merger.