Maybe it is really important to know precisely what an "L3" is, and since I've never worked for whatever company this is I'm unsure; but, FWIW, as someone who was just casually reading the comment, it didn't really come off as snarky in that sense, as if you are forcing all the strongest coders to get promoted then
by definition the people who are lower in the org chart are weaker coders, at which point the comment just "made sense" and didn't seem at all to offer actual "disdain" towards anyone except the promotion system (which maybe should itself be a reason to quit, but hopefully they are being paid well).
That said, I guess my explanation for the phenomenon--as someone outside such systems--is that in fact these organizations want weaker people at the bottom doing the grunt work as they are easier to control: they are all about figuring out how to build systems out of interchangeable cogs, under the premise that if you do enough build engineering then you can take almost any competent coder and get at least some contribution out of them (even if you end up needing ten times as many employees as a tradeoff).