Wild guess says that companies can't opt themselves out of sexual harassment laws with a simple clause in their contract.

Is that legally enforceable?

Of course not.

I don't know, but it was certainly scary to see.

No.

So when you sue them, you also get fired, if you hadn't quit already. I am pretty confident you'd find a similar clause in any company's employment contract, but even if it wasn't there it would be a safe assumption that any lawsuit brought against a current employer would result in termination.