The dealer mechanic did NOT remove the serpentine belt to inspect it. He just looked at it, which you can also do.
Belts have tiny cracks in them about a week after you install them. That’s just the way it is. If you replaced your serpentine belt every time you saw a crack in it you’d replace it every month.
They’re just trying to sell you a belt, which you don’t need yet. Don’t be angry about it. That’s what they do. They want to make some money.
I’d let them do a transmission fluid change, or flush, every 30K. If the fluid is dark, change it now. Automatic transmission fluid should never be dark or dirty.