"There's no downtime in cooking" is different from "cooking onions is not downtime". Though you'd still need to clarify "even if stuff is already chopped", I guess.
“Cooking onions is not down time” directly follows from “there’s no downtime in cooking.” If there is no downtime in cooking is true, then *cooking* onions cannot be downtime.
If cooking onions is not downtime, then it absolutely is an effective rebuttal to the general point of "I chop things [only] during downtime, for example when cooking onions".
"I only tell the truth, for example when lying" is irrecoverably self-invalidating. There is no second example that could nullify its effects. The argument is made invalid by that example specifically. (Not to mention, it calls into question the claimant's understanding of what it means to tell the truth.)
How is "there's no downtime in cooking" better than "Cooking onions is not downtime?" Both are assertions without backing evidence. For instance, it's possible for "Cooking onions is not downtime?" to be true (e.g. cooking onions requires active work) while "there's no downtime in cooking" is not true (e.g. cooking chicken broth involves downtime).
