If the guy does not even know the difference between threw and through and the fact that boiler is 30 years old, bet the bods at sales office were biting their nails in anticipation of quoting for replacement boiler.
There is a general understanding that with a safety critical component only the manufacturer's original part should be used.
I trust you do not have double standards about what you can do and what you expecty others to do.
How is a generic rope seal going to be different from one that has maker's name on the packaging. I assume you only use makers parts when you replace PRVs, pumps, Honeywell thermocouple (instead of generic) and gas valves.