It might be possible to do linguistic analysis (ie. What they did to catch the Unabomber) to compare the language they used in the review vs how someone usually write in things like performance reviews, manager evaluations, emails, etc
This could happen even if the culprit does get fired - as far as I know, there's nothing preventing you from posting reviews despite no longer being employed by the company.
Most of the time they won't need to go that far. The review itself will usually indicate which department a person's from and what their main gripes are. Sure, this won't work for FAANG or a factory where everyone hates "long hours, low pay" though the text analysis probably won't help much either, but if it's someone from an SME's dev team (n=10), the dev team manager is going to have their suspicions as soon as they see the bullet point about rescinded work from home policies or lack of attention paid to testing deployments...
(the non-trivial possibility they'll get the wrong person isn't going to stop fingers being pointed)