What the hell am I missing? Is it just that he got on Blackbeards crew
Okay so there's a few reasons to believe it. I certainly don't, because Mihawk is the entire point of Zoro's character existing. To undercut Mihawk would be a bold move from Oda and would have a very difficult time working out. IMO, if Oda wanted to subvert Zoro's end goal, he'd of spent more time on flashbacks between Mihawk and Zoro by now. Showcase that the full extent of their master/mentor relationship.
But as to why some people believe it and why people gas Shiryuu up as very strong...
Shiryuu was said to be the equal to Magellen back in Impel Down but even more lethal. Meaning, an encounter with Shiryuu is likely to kill you rather than an encounter with the poison man. Which is pretty nuts in and of itself.
Then you gotta remember what Magellen's purpose was—keeping every prisoner in line, including the Level 6 prisoners. When the likes of Jinbe, Crocodile and Ivankov would rather run away from you then fight you, ya know you're tough shit. Albeit they were on a time table and had to escape, I'd say take a look at the Rooftop.
Sulong Dukes could stomp Jack the Drought in a second. So in a group effort, the fact Jinbe, Crocodile and Ivankov considered that defeating Magellen would take too much time or the fact they might not of thought they could win is nuts. Even Ivankov alone, who was unafraid of Kuma, was defeated soundly against Magellen.
The final reason is that Teach's crew is the dark reflection of Luffy's. Shiryuu shares a common backstory with Zoro, both rescued by their captains from marine imprisonment and certain death. And that indicates, like Zoro, Shiryuu is going to be a cut above the rest of his crew. A real monster that usually gets compared to Luffy with remarks such as "And he's not the captain!?"
So yeah, 100% can see why some people would think Shiryuu is capable of killing Mihawk. He's not going to be another scrub.