ZD,
Ketchup seems to be considered a liquid, which is assur to reheat on Shabbos; it’s considered bishul according to some opinions.
A solid food such as chicken, is still considered a kli rishon (which causes bishul) when hot, according to some opinions, even after being transferred.
It follows that it should be assur to put ketchup on chicken, but R’ Moshe permits it, because there are opposing opinions on both issues, and we don’t need to be machmir on both halachos simultaneously.