I think my objection to the use of the term "Protestant Work Ethic" as a pejorative is that it can evoke bigotry on multiple fronts.
Most simply, it can be bigoted towards practicing/identifying Protestants.
More broadly, it can be bigoted towards "ordinary hard-working people" like my parents (who, I must point out, are in no sense conservative/right-wing in their politics or social values).
But it can be equally bigoted towards non-Protestants and/or people other than white Europeans/Westerners, by implying that a solid work ethic is uniquely identifiable with Protestants and white Europeans/Westerners, which of course is demonstrably untrue and insulting to people of other cultures/backgrounds.
I now understand that the root commenter was invoking the term as a byword for pointless busywork, as distinct from work that has meaningful outcomes.
But as I said in my parent comment, I'm not convinced that this is an accurate characterisation. At least I'm yet to see evidence for that.
I'm also curious about what people have in mind when they suggest that it shouldn't be considered necessary or important for most people to be working productively (i.e., for the actual betterment of society), in a world that seems to have limitless problems to be solved.
But I guess that's the beginning of a discussion about how we gauge the usefulness of the work people do.