Factual statements about bad employers are very much free speech. Judges aren't particularly fond of frivolous lawsuits. There are already mechanisms in place to quickly throw those away without wasting everyone's time and money.
So a random individual without the bankroll to defend is likely vulnerable, and this is a libel suit, so unlike a copyright suit, you can't solve it just by taking it down. You posted the allegation, now you must defend it. Yes, the truth is an absolute defense against libel, but you still need to defend it that far (through pretrial motions, discovery, etc.). Even getting to file for a dismissal is likely to be $five-figures with a good attorney, and you NEED a good attny. OFC, if you have a good case, a good firm may take it on spec, but....
And of course, how do you know all the postings on your site are actually fully factual and not exaggerated in any way?
Because, any honest employer falsely accused would be rightly very pissed off.
Tough problem.
You don't, but you smile and just delete the content. And then you've done your part as a platform. You're very happy to delete things when notified, and you do it promptly. Then you get to publish a very factual transparency notice.
Someone will have archived the page already? Social media is upset when they hear that the big company tried to attack the small independent site to take down this page that They Don't Want You To See? That's out of your hands, you're a neutral platform and you've done your part.
You just need to fold immediately, and you're covered by all the safe harbor neutral platform protections. Same as forums, social media, any website with user-generated content.
Now that won't stop an asshole from suing you over frivolous nonsense. But it does make it easy to throw their suit away - like you said, if you have the spite and money to follow through with a defense.
SLAPPs are only illegal in certain jurisdictions, not including federal or even many states.
and how is this hypothetical website you're hyping supposed to uncover the factuality of statements random people from the internet are posting?
Random people on the Internet have been at it long enough that there is plenty of precedent to establish that you can safely host a platform. But then again, I am not a lawyer, I am not your lawyer, and for legal reasons this is not the legal definition of legal advice, etc.
Don't publicize who wrote a review, but keep it on hand for subpoenas. You are probably not liable for defamation if you can identify who wrote the review - and they should be able to prove it's true. Ask a lawyer first of course.