I think it depends on the chemical used. Cyproheptadine is an inverse agonist, or a so-called "silent antagonist". A true antagonist usually increases receptor density but the mechanism isn't really fully known to claim with certainty for every drug. Keep in mind that cyproheptadine and most of the other drugs in that study are not "selective" - i.e they have myriad of other receptor activities such as histamine, acetylcholine, prolactin, estrogen, etc so it's hard to say what their cumulative effect on serotonin receptors is due to. If you Google for "serotonin antagonists" and look at the Wikipedia page that comes up you will see a number of selective serotonin receptor antagonists, usually on 5-HT2 subreceptors. Those when tested usually increase receptor density, so the expectation from a true and selective antagonist would be the same unless other evidence is available.