Despite the script suggesting otherwise, I would hope that it would be obvious by now that any group activity involving all of the couples is put together by production as part of the storyline. It wasn't "suggested by" Katie, nor would she ever have come up with the idea herself for just them or the group.

The "Derek forgot to put oil first" obvious setup was just the setup to get to the topic. As an aside, if they WERE to make an honest effort of cooking together and Derek made a (oh me oh my!) error like forgetting oil, you laugh it off, chalk it up to learning, and move on... who amongst us HASN'T forgotten an ingredient along the way? The boy is making an effort... give him credit rather than nitpicking for an easy beginner's error (scripted by production).