As a complete layman, when I tried tossing cold brew into the microwave it ended up tasting pretty gross. Like, jarringly so.
I won't pretend to know the science behind it, or perhaps I warmed it differently than what the parent poster does, but I definitely sympathize with the barista's hesitation in his story.
Yeah, there are still a lot of dissolved coffee solids in cold brew that get further extracted if heated. There are also a lot of volatile compounds that break down when heated. The idea that you can just reheat coffee, even cold brew and have it taste the same is just ridiculous.
