Error theorists are cognitivists. The sentence you quoted makes me think DaFranker is a noncognitivist (or a deflationary cognitivist,) he is precisely asking you what it would mean for U or D to have truth values.

By comparing them to abstract formulas, which don't have truth values ... as opposed to equations, which, do, and to applied maths,which does, and theories, which do...

When they are both trying to give accounts of what it would mean for something to be "right", it seems this question becomes pretty silly.

I have no idea why you would say that. Belief in objective morality is debatable but not silly in the way belief in unicorns is. The question of what is right is also about the most important question there is.