Avira,
According to the mareh makom you showed, it’s only the Tanya who is saying this. His quote from Eitz Chaim doesn’t back up his assertion (although I’m assuming he’s quoting it to further his point about their kavanos, not their nefashos).
Either way, it’s very shver. I don’t know anywhere else that says likewise.
Learn the gemara, it groups poshei umos haolam together with Yisrael. The burning and spreading of their ashes is their schar in gan eden.
Maskil, you haven’t been paying attention. He routinely misquotes or is outright בודה sources. Most of his hanachos are from his own svara