Thats total bullshit mate.
Loads of stuff has been considered 'underground' until it becomes popular.
Once it becomes popular, then it enters the realms of commercial pop.
I'd say Liam was a pioneer back then and influenced a lot of the big, early hardcore tunes of the day. He also never restricted himself to one or 2 styles either.
Chemical brothers, prodigy, underworld, etc were all considered 'underground'.
And one more thing.
Liam was never a front man, he always stood at the back and let the others hog the limelight so ive no idea where you got that from..