Calling someone a retard for believing something that’s obviously true is straight out of the flat-Earther playbook.

Replace “dumb kid” with half dozen of my boomer relatives, and you’re spot-on.

If anyone would know…

Definition # 4 is the applicable one here.

When multiple people politely point out that you’re wrong, perhaps digging in your heels and telling them to be better isn’t the right call.