DaveKat is not canon, said by Karkat himself I quote: " NO NO NO NO NO NO NO" - Karkat Vantas

A. Karkat could be embarrassed to tell John about his relationship with Dave, it’s not outside his character to do something like that.

B. Davekat could be pale, they obviously are very close and Dave was able to calm Karkat down pretty well.

C. Or they could just be buds, whatever

Either way yesterday’s upd8 didn’t disprove anything.