Either that's not correct, or the lawyers employed by these companies don't know the law because I [and other people] have definitely had designs rejected by PO

Either that's not correct, or the lawyers employed by these companies don't know the law because I [and other people] have definitely had designs rejected by POD sites for "trademark violation" when there was obviously no intent to "pass off" [which is what trade-marking is meant to prevent]. Someone just happened to have used a trademarked word in a design or its title or description. I suggest you have a trawl through that reddit link I posted above and read a few of the threads where people have posted examples of this happening to them.