True, but the protections still apply while he's POTUS. They might try suing him in four years, but that's a long time to pay lawyers to do nothing but wait. And even then, there's probably a lot of sympathetic judges that would toss any attempts to sue him over defamation.

IANAL, but I'd imagine there's also probably a statute of limitations on a case like that, which the POTUS protections might effectively defeat by running out the clock. It's a very niche area of law that's not be tested much by the courts. Virtually every President has some legal action begin during or after their term, but few have had pending legal action due to actions before their term.

Bill Clinton was one of the few, and it really dogged him and drained his finances, but he ultimately didn't face any criminal charges. Trump has a few cases involving contract disputes, but I believe he outright won those, like against Stormy Daniels.