The vid you linked though has the game running at 1080p low and 1080p very high, so you're both on 1080p, he's not on 1440
having a little scan to see at mo, but wont be apples for apples so to speak
guy using 1 stick ddr5 vs 2 (using 3090 and a 12900k) is CS go (not cs2 though) at 1440p ultra saw a 10% drop av fps, but 1% lows was a 37% drop and a 55% drop in the 0.1% lows
now below is another vid discussing using 1 stick rather than 2 or 6000 speed, and his findings are similar to above with a 10% drop is some games for av, so a 33 to 38% av fps drop seems high to me.
just saw another testing various games (the tech tank) and again he had dual sticks up to a 13% advantage (spiderman remastered) at it's widest with F1 22 having least (3% difference)
..and i've never tried running my rig on 1 stick so all i can go on is yt vids tbh
just checking your single stick you had running was on expo at 6000c30 speed though?