Not sure how you measure "skill" in singleplayer because everyones preferences are highly subjective but from my experience replaymakers spend a lot of time ingame. A lot. People considered me a prodigy when I was new and I've been more or less active ingame daily for the past 4 years. Meaning I've spent thousands of hours in singleplayer with the sole objective to become better and I still don't class myself as among the best.

sp and mp are affected diffrently by time as well. The sp scene is constantly becoming better and altough the playerbase is low at the moment meaning they don't get the recognition they deserve, we still have some of the best if not the best replaymakers active. While borders are constantly being pushed to become "the best" replaymaker the mp scene seems to be pretty consistant to me. I'm not big in mp but from my experience we have less active mp gods right now than we did say 3-5 years ago. Nuthug being my prime example

I don't think it's a fair comparison