Marbs was no less a bonefied NBA PG than pretty much 90+% of the quality starting PG's in the NBA today. I guess you could make a case they are all "combo" guards, though.
But no way was he a SG. He was a legit PG, and his best skill was penetration and finishing at the rim (or dishing before finishing).
I'd say he was more of a pure PG than many of the PG's in today's game, too. Idk if I'd call him some "pure point" guard, like Rubio, but nonetheless.
He was a legit 8assist/game PG. His career got torn apart, but you can't deny that despite being good for 20 a game, he was also good for 8 quality dishes that were quality in good part bc of his excellent speed and penetration ability and ability to kick it out.