Nobody's trying to push anything, lol.
1st. They're both held in similar viewing for Blackbeard, despite Shiryuu having been on the crew for 2 years now and Kuzan being a newer member:
2nd. Even in this fight, the two people to damage Garp are Kuzan and Shiryuu. And Garp gave both of them the same treatment.
3rd. The cover should speak for itself.
Rayleigh - strongest crewmember
Beckman - strongest
King - strongest
Kata - strongest
Killer - strongest
Marco - strongest
Zoro - strongest
Sabo - strongest
Bepo - Oda explains he picked Bepo due to strength
..........Logically, Shiryuu is the strongest after Teach.
There's nothing to push, Kuzan factually is the 3rd strongest member of the Blackbeard pirates after Blackbeard & Shiryuu, as the 10th Titanic captain. He's not an ally of Blackbeard's, he's not an associate, he's an actual crew member who is under Blackbeard's command.
Kuzan got weaker than his pre-TS self, Shiryuu on the other hand got stronger than his pre-TS self. I've said it for a while now, Shiryuu will be relative to Teach.