Nah, I wouldn’t mind that it was on backwards - simple mistake, sometimes our mind wanders during mindless jobs like installing 4 screws of a scope rail and our fingers work faster than our brain.
The disappointing part for me is that when you returned with comment that you ran out of elevation and couldn’t zero, that he ordered a different rail instead of just mentioning “no wonder you can’t, the rail is on backwards! Sorry for that mix up - gimme 5min I’ll flip that around and get it boresighted back, you’ll be good to go.”