That's exactly it. Android popularized it with its early versions when those phones had physical buttons. One of those buttons was the menu list button which op

That's exactly it. Android popularized it with its early versions when those phones had physical buttons. One of those buttons was the menu list button which opened the context menu. Eventually that button was removed and was replaced with in-app menu buttons. The remaining buttons became virtual, and then bikeshedded to triangle, circle, and square because usability be damned.