This guy does IT repair. DarkFrog is 100% right. The first and easiest check is with an external monitor, that’ll tell you if there’s any problems with the internal graphics hardware. And checking the cables is a (usually) easy thing to do that would have turned up the trouble right away.
Honestly, probably not malicious compliance so much as it was incompetent tech support personal. I’ve hired a number of folks from the genius pool and they can be pretty good when they’ve had a year or more of experience at the work, but mileage varies when you get folks who know the Mac software but aren’t hardware folks (I’m an old school PC hardware guy of course, though I had to support Macs as well in my last couple of positions).
