Spock didn't claim it was an absolute.
He said "After a time, you may find that having is not so pleasing a thing after all as wanting. It is not logical, but it is often true."
Kor
He didn't say "Sometimes having is ..." Or "Maybe having is.." He just said "having is...." without restricting it in any way. What he qualified is the possibility of the other guy finding out about it. His final "often true" is a semantic aberration. It would be like saying "Two plus two equals four' and "it is often true".
Or
A) I am telling the truth
B) A is often true.