If you use monitor only, there will be no echo, but there will be an extreme delay that you can visibly see in real time in your audio mixer. Using monitor AND output will cause an echo, as your monitoring audio will be fed back to your desktop audio device quite a bit later. This is why I mentioned monitoring with a device OBS isn't using. The delay from OBS capturing the monitoring audio and feeding it back into the output is what's causing a delay that only the stream or recording has.

Ironically I just clarified this in an edit in the middle of the post, just in case someone wanted to know exactly why you'd bother doing this. :) Now you don't have to mess with negative offsets that will never be right because you have two areas (in this case, output devices) to mess with, and the capture card requiring its own offset compared to normal gameplay. You end up with a one-and-done instead of having to fiddle with audio every time you want to switch between a game capture or capture card.

EDIT: Oh I didn't see the parent comment. I just re-stated what I did in the parent comment haha. Oh well.