Forget all the shenanigans the SH's did and Marineford, there were far other stuff going on at the time as it made BB's plan almost flawless.
- Marines going to war with WB, so therefore ID was less stationed with Marines
- Shichubukai went down to 5 members with BB appearing at ID and Jinbe refusing to partake
- Kaido and Shanks skirmish
- Luffy's shenanigans and prison break out
- BB using all this to manipulate the title of shichubukai
There's likely 1-2 more stuff i am forgetting but other than that Roggy can't use all the above to go break in as a shichubukai and come out unscathed. It's not like everyone in ID will just stand there and allow you to slap them in the face and walk out alive if not touched.
Its like Zoro being given the epithet "Pirate Hunter" yet is a Pirate.