Deeper than that, though, the INFJ's Ni finds it a little ridiculous that INFPs think any sort of "objectively correct" morality even exists at all, but that's another story.
Because that's the Ti equivalent of Fi's belief about objectively correct ethics.
And Ni would tell us that neither is truly objective at all.
Fi doms reject the idea of external morality being imposed on them, but they're laughably stubborn and unyielding
whenever anything threatens their arbitrarily imposed/subjectively validated conceptions of ethics. Most are not even remotely ready to accept that there could be anything wrong with these moral conceptions (Fi), but will still outwardly admit that there is in order to keep up an appearance of external flexibility
Hence the tragic hypocrisy that all too often results from being an NP: Ne leads INFPs to consider themselves as supreme examples of tolerance and open-mindedness...except when something violates Fi's values, and then anyone who disagrees is a BIGOTED INTOLERANT ASSHOLE AND A TERRIBLE PERSON! "Yeah I'm totally open-minded and ready to accept anyone else's ideas...unless they violate mine! THEN FUCK 'EM!"
Fi isn't really aware of the distinction between subjective/objective ethics
because it's so mired in its own internal conception of what is ethical and what is not. Since Fi requires no external validation for these moral beliefs, it ends up drinking its own bathwater through endless subjective self-validation
and that's very difficult for outsiders to deal with. (You can see the same thing with INTP's Ti obsession with "logic.")
It's funny how often INFPs will say that and then go right back into dogmatic insistence
that their ethical beliefs are the best as soon as anything threatens them (once they get upset.)
That's because you're an INFP and you don't understand the idea of ethics coming from anywhere but "the heart." Your entire conception of morality is contingent upon the idea that it comes from within, not from others, which is categorically Fi.