Does it? I don't see where he's making the claim that the reason you should listen to him is because he is a better designer than the folks at Medium. In fact, he says "[m]edium’s ho­mo­ge­neous de­sign works and reads well." The claim he's actually making seems to be about issues of openness, ownership, and authorial compensation, specifically that users are trading those things away for the fit and polish of medium's platform.

Unrelated, maybe it's a platform-specific problem? I found his site to be quite readable. Things like line-height and line-length appear to have been chosen with care, and he takes great pains to get all the typographical details right. The use of a "◇" to indicate links is...eccentric, but whatever.