If this happened no one would say he is as strong as Magellan. Shiryu simply accepted his penalty and went to jail after Magellan told him he is under arrest.
Magellan is the Warden, he outranks Head Jailer Shiryu, Magellan had the other guardians backing him up, and even if Shiryu beat Magellan, he would still need to deal with the rest of the guardians, then the Marines will go after him, so it was impossible for him to kill Magellan, other guardians and then escape from the Marines.
Ivankov simply said Impel Down was impregnable because of both Magellan and Shiryu, and he said Magellan and Shiryu were equally powerful.
That's Base Shiryu without even a DF power.
Adding a powerful DF ability to that base, would definitely make him Admiral level minimum.
Being put into Level 6 is a feat, Shiryu was put into Level 6 despite not even having a bounty. That's how dangerous he was. People like Daz Bones were in Level 4.
BB's Level 6 crew was called as ''legendary'':
Then Shiryu was confirmed as No.2 of BB pirates with this:
That means he might be potentially even stronger than Aokiji, who is currently a member of BB pirates. If Oda was hiding Aokiji as BB pirates member at the time, then its still impressive to be the strongest amongst the other strongest Level 6 crew members of BB.
BB made all Level 6 prisoners to kill each other, and he only took the
strongest members of those legendary pirates:
Shiryu is now confirmed as the strongest Level 6 prisoners with that cover.
Also, Oda himself give credit to BB Pirates, by saying that he would join BB pirates other than SH pirates ''if he was strong'',
''Q: If you could join any pirate crew in One Piece (except for the Straw Hats), which one would you join and why?
E.Oda: Hmm. Except the Straw Hats… I think it would be awesome to be in Buggy’s crew. They seem to be very carefree, and it would be really fun because even the subordinates can make fun of the captain. But if I was really strong, I would want to be in the Blackbeard Pirates.''
That's why BB pirates are special, even if you didn't understand from a former Admiral Aokiji joining them.
BB pirates will be different than the other Yonko crews, except maybe for Shanks's crew, Shanks's crew could be stronger than other Yonko crews as well.
And, another similarity with the Admirals, Oda creates Admirals by being inspired from the Japanese movie characters. Zatoichi etc, Shiryu is based on a Japanese character as well, Yasunori Kato. Very powerful character. Its also the base for M.Bison from Street Fighter series.
