Roger (with Garp's help) defeated Xebec / Whitebeard. While Garp saved the Celestial Dragon and the slaves, Roger destroyed his greatest enemy who was Xebec, who was captain of Whitebeard. Not counting the likes of Kaido and Big mom yet because they weren't Yonko level according to Sengoku. As well as Shiki wasn't in his prime.
Years later Roger fought against WB for 3 days, ended in a draw.
Roger has 1 win 1 draw.
WB has 0 win 1 draw.
Roger easily overpowered someone as powerful as Oden. Oden was no match for Roger.
Shiki was mad and he attacked marineford when he heard Roger is captured, he didn't believe the marines can capture someone as powerful as Roger. Then he said the marines are garbage in comparison with Roger.
Buggy used Roger to hype Whitebeard's strength, and he said Whitebeard was the only man who can fight equally with Roger.
They use Big mom argument to show that Roger didn't become the PK by using his strength, whoever says this is either ignorant or trolling because Big mom herself said this;
BigMam: That accursed girl……!! / If only she had obeyed me and got married back then…!! // Unfathomable power would have been mine…!!! // I would have crushed Kaidou, and Red Hair, and even Whitebeard!!! If it weren’t for her, by now.
I WOULD ALREADY BE… // …THE PIRATE KING!!!
--- CHAPTER 847
*Big mom specifically says she could become PK by defeating the other Yonko.
*Luffy said the same thing to Fujitora.
*Chinjao said the same thing to Luffy.
*Mihawk, at the beginning of the series, says that being PK > Surpassing himself.
The evidences are clear; PK is a strength related title, you can't be a PK without being WSM or WSC first.
E.Oda showed us first that Roger was equal with WSM, defeated someone as powerful as Xebec while Whitebeard was also helping Xebec, he also showed us that he could overpower someone as powerful as Oden easily, and Shiki specifically believes that no marine can rival Roger's strength, and Buggy uses Roger to hype Whitebeard's power.
You can't become the PK without being WSM or WSC first.
But that doesn't mean you can be a PK by simply being WSM or WSC. It's WSM / WSC + more by finding the last island.