One thing to consider... Some people do not understand the generators who's model numbers end in "i"
Now, I have seen portable generators (IE: COntractor models) that run 3600 RPM, priod, that is the idle speed, it's also the full load speed.
The fact the service tech said "No more than 3600" Tells me he does not understand "i" models.
Some run a sub-multiple of 3600 (IE: 1200, 1800 2400) and by either gears or other means put out a 3600 cycle per minute (60 HZ) signal.
But on INVERTER generators the engine speed has absolutely NOTHING to do with the output frequency, no coloration at all. except at ZERO. (if the engine is not running the output frequency is also zero, as is the output voltage)