There is no meaning to this statement if you can't spell out just how well Magellan compares to the top tiers. Where does it say Shiryu's been training for the last two years? The guy's in his 40s and probably maxed out his base stats before joining BB. His only new power-up is invisibility, which, depending on how well he uses his abilities, makes him too dangerous to fight for about any character in the series. That doesn't mean, however, that he is as strong as the top tiers.
If you're to ask me, Zoro is already the strongest subordinate. Shiryu will just be a tricky opponent to fight. I doubt he could exchange blows with KoH, or even non-KoH CoC based attacks for that matter.