Honestly, to me it seems the way it originally is in the novel is a dumb callback to Boba being the son of Jango even though he was technically only a clone. I don’t believe Palpatine would claim a non-Force-sensitive clone as his son, personally, so I wanted to lean more heavily into him being a clone over a son.
This might make more sense. It’s one thing for Kylo to describe Rey’s father as the son of the Emperor in order to streamline the revelation, but Palpatine indirectly reivindicating him by referring to Rey as his ‘grandchild’ doesn’t seem something like he would do (if anything he should be ashamed of how useless the clone is for his plans, except for continuing the bloodline). So for me it seems better from a story and continuity aspect to refer to Rey as the daughter of Sidious.
I only said the Unkar thing because it seemed strange for me that Rey’s parents, now retconned as heroic and good-natured people in TROS, still sold her to a really awful guy.