No, it wasn't. Mazda, Mercedes and Jaguar all had to stick with 1990 spec cars - Mercedes and Jaguar because their 3.5L engines, as pretty much everyone else's, weren't ready and kept exploding - and Mazda because, well, it was a rotary.
FIA/ACO decided that, fine, if you don't have a reliable/compliant engine for using a 1991 chassi, you can still race with a 90 spec car, provided that you add those 100kg of ballast. This was mandatory: if you were to stick with the old car, in order to prevent a potential engine DNF early on in the race with the new one, you had to do so with 100kg more (or, in Mazda's case, you couldn't even start the race with a 91 car + non-3.5L combo). Peugeot, the only manufacturer to actually qualify and race with a 3.5L car were free from this.
So, if the ballast was mandatory, and Mazda was allowed to just ignore it, I think it's pretty clear that they ran that car outside the rules that their class rivals had to follow. Mercedes jumped to a huge lead (4, or 5 laps ahead, IIRC), until being hit by realiability issues, and Jaguar never had the pace because, you know, 100kg is kinda heavy...