I don’t know what the official canon explanations (retcons within retcons) are, whether Snoke was a literal puppet or if Kylo could literally hear Vader’s voice guiding him, but I think that’s how the line reads in this movie. It’s misleading at best, and a retcon at worst.
When Kylo would ask the Vader helmet for guidance in TFA, it was clearly meant to be more of a personal reflection and internally meditative sort of thing, not him literally being spoken to by Darth Vader’s actual voice.
To me Kylo’s relationship with Vader in TFA seemed exactly like this. A fanatical worship of Vader’s legacy, which he feels he must carry on (“I will finish what you started”). In TLJ he gives up this quest all together, because as Snoke tells him, he’ll never live up to Vader. So he destroys his mask and sets out to forge his own legacy. In this movie he finds Vader’s former Master, who tells him he can not just live up to Vader’s legacy, but surpass it (“become what your grandfather Vader could not”). So he picks up that quest again, reforging his mask and placing Vader’s helmet by his side again.
Not the most exciting overarching story after TLJ, but it can at least work if you don’t imply that 1.- Palpatine was also talking to him as ‘Vader’ (which leads him nowhere, as he gives up on that quest) and 2.- Palpatine was also talking to him as ‘Snoke’ (the man who litteraly tells him he’s no Vader). It’s honestly baffling how obsessed this movie is in making Palpatine be directly involved in everything from the previous movies. Making Palpatine’s relation on Kylo less direct (he was the Master of Vader, who molded his whole Kylo Ren persona. He was involved in some way with the creation of the First Order he commands, and he “made” his former Master) works far better.
The line could still be read as meaning he trained Snoke and made him who he is, while also hinting that it may be more literal.