I don't really see it TBH:
- Borsalino has dealt some minor damage to Luffy
- Luffy hasn't hurt Borsalino at all so far
- Borsalino has been trolling Luffy
- Borsalino ignores Luffy to deal with Vegapunk
It really doesn't look like Borsalino is treating Luffy as a threat/taking him seriously.