Of course it isn't guaranteed. I have said so many times myself. However "hate speech" is often used to mean "conservative speech with which I disagree and don't have a rational comeback."
Specific rules against specific speech that runs counter to the ideas of the site are reasonable: On OCDO we are not supposed to bash LEOs in general. On almost any site flaming members is not allowed. On almost any site racial epithets are not allowed. Etc.
However, "hate speech" is a nebulous term that allows arbitrariness on the part of admins and mods. That will allow facebook to skew political speech in any direction they want it to go. That is expected for a political site, but not for one that ostensibly welcomes all.
Yes, they have a perfect right to do that. I will just point out the flaws in their policy, express disdain for it, not participate in their site as a result, and hope that I influence at least one other person not to go there.
I prefer to go places where I can express my views within reasonable limits that do not inhibit the free exchange of ideas.