It's not Da Vinci who remodeled the hidden blade, it was Altair after the events of the original AC game. I didn't play newest AC games so I don't know what's the explanation for their own hidden blades, but this viking's blade looks wildly different and not so hidden. Might just pass off as a cultural difference.
Would be fun if there are some references to Vinland Saga in the new AC game. I might try it out.