The online for seamless coop isn’t actual online, it’s limited to the functionality of the mod and doesn’t use the official servers. You can in principle get cheaters on that, but most people have it setup to only connect to friends. And when you are using mods you are probably just having fun and playing for the memes instead of being a hardcore PvP player.

I personally love seamless coop, I had hoped FromSoft would have made the coop like that from the beginning. So happy mod makers are putting in the time to make it work. I’ve played over 100 hours with my brother and had so much fun. I can recommend mods like The Convergence as well, combined with seamless it’s so much fun.