It miffs me when there's a blanket statement like that. I set up a warranty with HTP. They asked if I was the homeowner, you have to state that you're the builder and they seem fine with that.
I don't know exactly what "qualified" installer means. You could be a qualified plumber and have never seen a tankless before. Don't mean the system is going to last longer or not short cycle with hand washing. You could also be the best plumber but no gas ticket, well they're not qualified to put one in are they? Or be a gas tech that slap some plumbing together with sharkbites, that's qualified right?
I think that statement about maintenance is in there maybe so homeowners that never descale, make a warranty claim, and say they've descaled every year. Well there's no proof at that point. Vs receipts as proof from a "qualified" technician