It's easy to prove that you didn't get paid though. The company would lose in a second, assuming the claim is true. Defamation is only defamation if it isn't true.

Surely they'd know that they didn't pay the employee writing the review...

That being said, the whole scenario seems absurd. Most countries have easy ways for employees to file claims of non-payment and companies are forced to pay up.. I've never heard of a company not paying staff AND getting away with it unless they're bankrupt.