But why is this pleasantry about "luck" and not something else? You don't hear employers say "keep studying" or "we wish you a good effort" or "we hope you build cool stuff".
Isn't the employer referring to luck an admission from the employer that interviewing has a lot to do with luck?
Can you explain what you mean by "really enforceable"? Does it mean that they often don't get enforced, but are legally binding -- or does it mean they're illegal?