Why is it bad to drop revs fast?
 

The orevious owner of the car told me it was bad to rev rotaries up and let them drop without load on them. Is this true? And if so why?

the previous owner is on glue..

He said itll burn up the rotors. And yes he wasnt very knowledged and talked out his ass a lot

It does no more harm than doing that in a piston engine. And remember, there is no reciprocating mass and the rotors are turning at 1/3 the speed of the eccentric shaft...rough equivalent to the crank in a piston engine.
The caveat to this is that a lot of guys will claim that red-lining a rotary is good for it and proceed to cob it routinely for no discernable reason. I don't agree with that either.

So are the rpms based off excentric shaft speed? Or rotor speed? And no reciprocating mass? What about the rotors and excentric shaft?

RPM is based on e-shaft speed. Rotary engine power pulses
And yes, no reciprocating mass. In a piston engine, the pistons are going up and down...reciprocating. And to change direction their motion has to stop. Not to mention valves going up and down, springs compressing, rockers, cams, and usually more bearing surface. In contrast, a rotary engine has none of that. And everything in a rotary engine turns in same direction.