Is not responding to an infidel parent's request to prepare food for them while they are fasting for other than Allah considered disobedience to parents, or is not responding the correct thing to do?

You mentioned previously that your parents claim Islam. The default is to rule that they are Muslims and not to accuse them of fasting for other than Allah until proven otherwise.

Therefore, we advise you to be dutiful to them and not to accuse them of disbelief without evidence, and to stay away from obsessive thoughts (waswasa).