Yamato No Orochi =/= Hydra
If Oda want's to add traits from the Western Hydra to his Yamata no Orochi, he will. Just like his Hou-ou has fire and regeneration properties stressed because of the Western phoenix (a common evolution of the classic figure in modern, popular media), or his Daibutsu sends shockwaves with his "palm of Buddha", or his Seiryu has the whole pack of elemmental, weather powers attributed to "ryus" as a generic figure.
He is a modern author, not a traditional one, and his depictions of mythical creatures are commonly either modified or affected by globalization, just like in Marco's case or even Kaido's fire breath, which is way more common among Western dragons. Let's not pretend like Orochi can't regenerate his heads only because his fruit isn't literally a hydra; that's not how this manga nor his author works.