First, as for why the lightning disintegrates him in this movie, he was putting all of his UnLiMiTeD power into it. In ROTS, he was clearly creating a show for Anakin in order to tempt him.

Second, I wouldn’t mind the change to the word “become” (even though my TLJ edit removes Snoke admitting to bridging their minds). I say that because obviously at some point they’ve become a dyad in the force, I just prefer to leave it up to interpretation (my personal idea is that it started in the interrogation scene).