Hmm, right if munocat was saying :

They had eleventy billion percent market share"

you'd know it was hyperbole; an exaggeration. Now in this case if the OP types:

"they had about 98% market share"

then that could still be hyperbole. 98% is a high number, the sort of thing you'd reach for when exaggerating.

Saying "literally about 98%" is supposed to quash any exaggerations and send a message to take the sentence at face value.

The times when people misuse literally usually go something like :

"Yea the boss was so mad at him he literally chewed him up and spit him out"

^ that's when people misuse literally.