I would agree the "don't blunder" and "punish opponents blunders" are harder than endgame knowledge. However, knowing the basics of endgames is actually important to closing out games. Specifically, knowing KQvK, KRvK, and the "ladder technique" is important.

Without any tactics "take free pieces" probably only gets you to around 1200 (chess.com), but if it includes knight and pawn forks, skewers, pins, and discovered attacks it can get you to 1500. Playing perfectly every game is hard though. I would recommend having more chess knowledge than just those 3 rules before you really try for 1500.

Maybe it's just me, but in an endgame where (for example) it's just kings and a bunch of pawns, even a two pawn advantage is easy for me to lose—I pick the wrong pawn island to support with my king, and I've already lost. Knowing good heuristics in these sorts of situations is something that takes active study IME—"don't blunder pieces" is not sufficient, at least for me.