yes, nothing exists outside the omniverse (of marvel), so when MJJ goes to the nothingness there isn't concepts he can manipulate, and thus he is powerless.
The real omniverse exists outside of the Marvel concept, so he can be taken outside of his "omniverse" and warp reality as long as he wasn't transported to a void realm where reality doesn't exist by our definition. Unspace, theplace he was taken to be killed by the Fury is a Marvel construct, but by the strictest definition of the omniverse could not possibly exist within the marvel omniverse (multiverse) because there is no reality, so it would not be mapped as such. I'm speaking as if he was transported out of marvel into lets say , DC, where there is plenty of reality not dependent on the Marvel "omniverse".
i must have misunderstood you there, i agree with this.