Zoan piece lol.
Unfortunately, I think Oda has finished the really good ideas and so it's way easier to come up with new powers using the ton of possible mythical creatures.
Well, not really lol,
By this logic (quoting a joke SBS from 30 volumes ago), I can say it's a reference to the supreme power of Ener (lighting), which was said to be invincible --> strongest; or I can say it's a reference to the dragon, the strongest creature. Etc.
If in this chapter a dialogue doesn't say or imply anything, it is dubious.