1000 chapters into the story and people still don't understand terminology. Some of it is on Oda though, can't lie, his explanation and omission of things usually make no sense.
Ryuo = Haki, but a specific type of haki. Barrier haki. When Wano natives talk about Ryuo they are talking about barrier haki.
Luffy who had a grasp on the basics of armament haki needed to be tutored to achieve barrier haki by Hyo.
Luffy who was taught the basics of haki by Rayleigh, and has successfully utilized the said basics by being able to coat his arms/legs etc and interact with logia types had to be taught how to achieve ryuo. Ryuo is different from hardening, regular haki.
Luffy himself even notes that while practicing. A coated by hardening fist extends to hit a fodder and Luffy says 'No, that's not right.. That's just
normal armament haki'
He could not achieve what Hyogoro did, which is barrier haki, or Ryou.
During his training however Luffy went a step beyond barrier haki and achieved internal damage haki.
Again Hyogoro tells us that.
Hyo differentiated between the barrier and the internal damage haki, with the barrier being the stage 1 of advanced haki, and the internal being the stage 2
Right side of the panel above = barrier haki.
Left side of the panel above = internal damage haki.
Luffy went a step beyond what Hyogoro wanted to teach him. Hyogoro could teach him the part on the right, which is barrier haki OR
RYUO.
In regards to Zoro, he has been capable of Ryuo, at least subconsciously from Alabasta.
The concept for BOAT is repeated word for word by Hyo when talking about Ryuo aka haki
I've made a diagram below, it should be easy to follow
Why do I say subconsciously? Because at that time Zoro did not understand what it was, didn't master it yet and couldn't bring it forth voluntarily.
That said, to return to the topic of the thread.
EoS Judge's son shouldn't be capable of scratching Kaido's balls, let alone current Judge's son.