Interesting....so I looked into it a bit more and the piece that plays is officially named
Palpatine's Teachings. So yeah, not Plagueis theme. Buuut....I'm going to put my money in the bucket that Williams scoring both the way he did and the manner in which they were used can't be a coincidence. At least I hope. I kind of like the idea of Snoke being Plagueis.
The question would be "What the hell was Plagueis up to?" in the originals. I'd be pissed if it was some stupid puppet master story....he was behind the scenes the whole time. What I would LOVE is if he was truly afraid of Palpatine, so he stayed hidden until he died.