And EVEN Prime Shanks, this is 1B Shanks who stoled Nika fruit.
Bounties aren't reflective of power either.
Zoro and Sanji still have lower bounties than King and Queen.
This version of Shanks can already used advanced Conqueror's Haki. He is likely already strong enough to qualify as an Emperor, he just doesn't have the crew, resources, alliances and territories to warrant it.
We don't even know what he's done so far to justify it.
People need to understand, the definitive Roger Pirates flashback will probably delivered by Shanks himself before delving into his own definitive flashback.
There is so much we don't know yet.