Doesn’t change much, his base stats were always strong as they qualifies him for a commander position in the WBPa. He also scarred Shanks in base, who as a yonko was still afraid of his power. He had a month to get used to the Gura and already fought Sengoku with it. Add to that his crew then it’s a no brainer.
They weren’t yonko simply because they haven’t amassed the territory required.