Sounds like the mechanic at that shop has an incomplete understanding--probably just knows that there's a variation of rake for forks and head tube angle for frames, but doesn't know enough to know what typical values are to actually determine compatibility. Maybe he's assuming that the frame is more track-ish with lower trail than it actually is, and thus not designed for a road fork?

If you get a chance try to explain to him or direct him to why he's wrong, and if he's too stubborn to realize he was wrong, talk to his manager. It's bad business for wrenches to be outright wrong about bikes.