I am also not a lawyer, but I've always heard that telling the truth is a bulletproof defense in libel/slander/defamation situations. Some employment contracts have non-disparagement clauses but I think they're fairly rare (kind of a red flag) and at least where I live they can't prevent you from truthful griping about working conditions

As I understand it, this might be true in America, but in Japan, "it's the truth" isn't a defense against libel.

In the United States, truth is an absolute defense against libel. You are correct that it is not in other countries, but this is someone in America (probably an American) talking about an American company.

That's wild. How does that work out legally? A company can sue an employee for telling a damaging (but uncontested) truth?

It works if the law says that reputational damage trumps the fact that the statement causing the damage is true--or can't be proven to be false. Although the UK has made some changes in recent years, it's probably the best example in the West of a country where truth isn't necessarily a defense against a libel suit. https://en.wikipedia.org/wiki/English_defamation_law (And somewhat ironically given that US law has its roots in English common law, the US swings heavily on the side of free speech.)