Those other three are all evil cultivators, so I don't think their words should carry any meaning. If a righteous cultivator makes a promise, while having the intention to do what they promise, and then breaks that promise, I reckon it would lead to a heart demon. But if an evil bastard promises something, is it ever anything but empty words to gain profit? It shouldn't lead to a heart demon because lying is a way of life for them, so exceedingly rarely would they also think they must keep this promise when they are stating it aloud. For example Yang Kai mentioned that the offer from the Purple Star geezer to make Yang Kai a guest elder was nothing but empty talk to delay Yang Kai until the destructive light disappears. So, he could have killed Yang Kai, ignoring his own offer, if Yang Kai had decided to accept the offer and just waited.
Naturally none of this means Yang Kai shouldn't mock them afterwards.