It feels more like Katakuri who was able to change shape despite not being a logia.
The Gomu Gomu no Mi might be a Special Paramecia as well. With how much alike Luffy's and Katakuri's powers were depicted, I wouldn't be surprised.
And besides, King and Sasaki did weird things in their fights that they shouldn't be able to do based on their fruits, so Kaido's statement doesn't seem THAT important to me.