That was written during 19 BBY when Palps discovered the Force Storm technique. By the time of Dark Empire, he would have perfected the technique.
The true irony is that he DID perfect it by Dark Empire, he just went into his typical Dark Side high(REALLY can't blame him as the Dark Side is like an extremely addictive drug and has been stated as such, Palpatine would be on the Tony Montana route) and went after everyone else and fucked around too much(Which helped caused his defeat).
So it doesn't even make sense for either Disney Dark Sidious(Who used Force Storm against Rey) nor Legends Sidious(which nothing states he cannot use Force Storm on someone nearby amongst many other tools like the former) to effortlessly dispose of Frieren past absolute nonsense.