It's not "you are Yoshinaka-sama", it's "you accepted me even when I'm half oni just like Yoshinaka-sama so I will serve you like I served Yoshinaka-sama, minus the waifu part". It's like Tamamo and Guda, Guda doesn't care if she has Beast mode just like Zabi in CCC, so she will serve Guda like she served Zabi, minus the waifu part.