You have asked two questions:
Question 1: Is toCasefold(NFC(X))
binary equal to NFC(toCasefold(NFC(X)))
?
The standard doesn't explicitly answer this question. (I would expect the answer is yes, that case folding does not affect normalization, but I have no proof.)
Question 2: What is the Unicode standard telling me in the quote?
The standard is only saying it is not necessary to do case folding again after canonical normalization. In other words, canonical normalization (to NFC or NFD form) does not change the case of any characters from uppercase to lowercase or vice versa. This doesn't answer your first question.
It is not saying whether or not it is necessary to do canonical normalization again after case folding.