As I recall, he told Ace that after he killed Thatch. He didn't tell anybody his real reason for being on Whitebeard's ship until after he already murdered Thatch.
He was quite happy using the familial relationship that the Whitebeard crew had to his own advantage.
In chapter 576, you have Whitebeard clearly stating that Teach was formerly considered a brother and a son on that ship. Teach even desperately brings up being "family" as a hypocritical reason as to why Newgate shouldn't kill him.
Teach clearly was fine with being treated as family. Everyone on that ship treated him as family. Hence his betrayal is absolutely genuine regardless of Teach's own take on the matter.
Even if you want to discount the "family" aspect of it, he was undeniably a trusted crew mate. The betrayal is again, undeniably genuine.
First of all, she is obviously trying to recruit Kaido onto her side in that scene, so being the conniving crone that she is then she is naturally going to smack talk the captain in order to get the powerful new rookie to side with her.
Second, all she says in the translation there is that Xebec is "good-for-nothing", "don't trust him" and "If you have a problem, talk to me".
She is talking about Xebec in that scene more like he is a lazy idiot that can't be trusted to help somebody with a sensitive problem than somebody who is going to blindside betray you over a piece of treasure. Lmao. I got no idea where you are getting Linlin treating him like Judas II from.
