There seems to be a clear intuitive problem with the conclusion that 13 year olds can't consent to sex with each other when the premise is that it's wrong for a 13 year old to have sex with a 22 year old.
Do you agree that it seems troublesome? Maybe stating it in dialogue form will better illustrate:
"Why can't 13 year olds consent to sex with other 13 year olds?"
"Because a 13 year old can't consent to sex with a 22 year old."
It just seems unrelated, a bit of a non sequitur. Consent between persons aged 13 and 22 feels questionable because there's a large age gap. In cases where there's not a large age gap, that argument is suddenly dubious.
There are reasons for why a 13 year old can't consent to sex with a 22 year old. Use those to derive the supposed wrongness of two 13 year olds having sex, otherwise, it's simply a misplaced analogy.
No, the issue is the same in both cases: A 13-year-old can't consent, full stop. The age of the other party doesn't somehow make the 13-year-old more or less mature.
May those who accept their fate be granted happiness;
Those who defy it, glory!