I think they mean you can't, "mute the ad, and scroll down and read the comments (no longer seeing the ad, because one is scrolled down and reading the comments

I think they mean you can't, "mute the ad, and scroll down and read the comments (no longer seeing the ad, because one is scrolled down and reading the comments)", not because one can't mute the ad, but because scrolling down now, instead of showing the comments, shows a bunch of video recommendations.