I can guarantee I've mutated austere in a vanilla desert biome using an apiary and nothing more than three soul frames - and the frames are a maybe, most base forestry breeds I didn't bother.
In this current game, I've never made a thermionic fabricator, much less alveary heaters.
In my prior game I'm fairly certain I used the same setup in a vanilla extreme hills (with Both_2 tolerance) but wont swear I didn't travel to a desert. I do know that I got 100% of the breeds available at the time other than dyes without ever making a mutator, and frankly don't see the point of them.
Sinister and Fiendish do require the nether, and one or the other is required for Frugal, but neither Frugal nor Austere require the nether to mutate. Or even a desert.[DOUBLEPOST=1365866863][/DOUBLEPOST]
Also, I really hope your talking about not using any temperature modifying devices like an alveary heater, becuase those can be used to create Austere bees. I've only been talking about a natural mutation.
Btw, if you were correct here (you're not) then it wouldn't be a biome restriction. Fiendish will not mutate outside of the nether no matter how many heaters you use.