Mend can't fix magic.

This spell can physically repair a magic item or construct, but the spell can’t restore magic to such an object.

Magic smoke is magic smoke.

Something something simple repairs... DM said no

They’re making a joke, riffing off the “you let the magic smoke out” joke used in hardware repair. The “magic smoke” is when the chip fries and needs to be replaced, and mending can only physically repair magic items.