All true, but anyone putting up a site would need the bankroll to handle such a lawsuit through to winning, and then be able to collect on the SLAPP judgement (which the non-paying employer would likely have put up obstacles to collection). That's the big problem with lawsuits — you start out in a losing position just because of the cost to defend.
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.