"Humanitas" here ought probably not be read as "humanism," but rather as "kindness, gentleness, or philanthropy." Hadrian is telling people he's a benefactor to his subjects, which is in keeping with what pretty well every emperor has said about himself since Augustus. "Libertas" does, in fact, mean "liberty." It is a heavily connotative term in the imperial period, conjuring up at once the idea that the Romans are not slaves to Hadrian, nor is he their authoritarian despot (in the manner Greeks and Romans liked to imagine Persian kings lorded over their subjects). It also suggests that Hadrian is accessible and able to hear free and frank speech (within reason). "Felicitas" finally just means good fortune. The model of ruler implied here: a benefactor, accessible, willing to listen to criticism, and creating good fortune and prosperity for his subjects, goes back to Alexander the Great and his immediate successors. There is, in other words, little new about it.

Nor is their a great deal new in Marcus Aurelius' stoicism. The Roman elite had a long history of dallying with that particular philosophical school. Ciciero, for instance, was deeply influienced by stoic thought, and Seneca the Younger (best known to us as a stoic philosopher) was an advisor to Nero during the earlier (slightly more stable) years of that emperor's rule.

All this to say, reading humanism back into the second century emperors seems to me to be anachronistic. Nothing here suggests that they are not operating within traditions established centuries prior to their respective rules. Nor do any of the central tenets of modern humanism (a prioritization of humanity over the divine, a belief in the inevitable progress of humanity, a notion that humans have inalienable rights by virtue of being human) map very well at all onto Roman beliefs.