I don't think anyone is claiming that this describes all instances where someone calls something immoral.

It's merely calling attention to two interesting phenomena: (1) envy can make people regard something as immoral when their real problem with it is that others can have it and they can't, and (2) even when someone has actual principled reasons for disapproving of something, once they are in a position to take advantage of it themselves they are liable to forget those principles.

(Perhaps those are really the same phenomenon, deep down. But they feel different enough that, e.g., it took me a while to figure out how anyone could think the quotation had anything to do with the idea that "power corrupts" because I was initially thinking only of #1 while cousin_it was referring to #2.)