Lol Haki on sword is literally advanced application of Haki per Luffy's own words. What Luffy called advanced Haki was Rayleigh channeling Haki out of his body in form of shock wave. Then when Hyogoro was explaining that phenomenon, he called it Ryouou which is also swordsmen's ability to channel excess energy out of their body and becoming one with their blades having ability to cut everything and nothing.
So not I am not using speculation to support my speculation. Hell I am not even speculating, I am literally stating facts from manga word for word. We were told this in manga right here
Read this fucking panel carefully lol, you maybe able to understand. "The thing you seem to call Haki, its called Ryouou in Wanoo, the word signifies it FLOWS" the keyword is flow, and then Luffy is trying to making Haki flow out of him (Aka use Ryouou) but fails to use it when there is Haki only on his fist but it doesn't flow. And in the last chapter it was said Enma sucks Ryouou out of user's body. Again focusing on flow of Haki.
If you still don't understand why flowing Haki out of your body and putting it on swords is advanced application of Haki (Aka Ryouou) you need to try this link and make your reading comprehension better
https://www.grammarbank.com/reading-comprehension-test.html