This possibility doesn’t undermine the argument. Perhaps someone can compare oneself both to people thought of as more successful and people thought of as less successful. The author’s conclusion concerns what happens if you stop comparing yourself to anybody at all.

The conclusion is not a recommendation that you shouldn’t compare yourself to others. It’s simply a description of what the author thinks will happen regarding self-acceptance and acceptance of others. Other benefits from comparison have no impact on this conclusion.

One assumption is that NOT being self-disparaging implies self-acceptance, and NOT being dismissive of others implies accepting others. (C) wrongly turns this into the idea that being self-disparaging implies NOT self-accepting, and being dismissive implies NOT accepting others.

If this possibility were true, the conclusion does not follow. Refraining from comparisons could not be expected to lead to avoidance of self-disparagement and dismissiveness if there were other things that caused these feelings. So overlooking this possibility is a flaw.

The author is open to the idea that not all comparisons involve people you think of as better/worse. The author simply asserts that if you don’t compare yourself to anyone, you’ll avoid things that we know result from comparing yourself to people you think of as better/worse.