I wasn’t even aware they were chanting actual words, but I’d be down for it to be Sidious.
Curious that Sidious got name-dropped in TLJ but was only referred to as Palpatine/Emperor in the movie that actually featured him.
It’s not really all that noteworthy since both Palpatine and Sidious are names that were only used in the Prequels, so either way it’s an acknowledgement of them. Then again, the Emperor had always canonically been Palpatine and was established as such in additional material, so maybe Abrams called him that not 'cause of the Prequels but 'cause it’s what was on the box of his childhood action figure or something. The same logic applies to the use of the word Sith.
I was actually fearing they wouldn’t call him Palpatine in this movie because of the “ignore the prequels” mindset.