Bruh
Seeing as they're both zombie viruses I think it's worth noting that while L4D survivors can't be turned into zombies by their verse's infected, (At least I don't think they can, correct me if I'm wrong here) a single bite from a PZ zombie will inevitably lead to infection & subsequent death just like any other person who isn't immune to the airborne variant, (With scratches & lacerations also having a good chance of infection, even if it's not guaranteed in that case) meaning the PZ players have no special resistance to the kind spread by zombies compared to normal people and are just as vulnerable to being turned by their world's zombies as someone with no disease resistance
I don't think we have any reason to assume otherwise, it's an airborne disease that spread across and wiped out the entire county the game takes place in, there'd be no way for the player to avoid contracting the airborne variant, they just so happen to be immune to that one specific strain. While L4D infected are generally more dangerous than PZ zombies, the latter can far more easily infect the PZ player, with a single bite being guaranteed death and scratches & lacerations being very real chances (And fairly high for lacerations) of death, disease resistance be damned.