I don’t think they all used “alumnius.” At least one of the photos was captioned “alumni,” and articles have referred to other seniors’ page saying alumnus or alumni. This might have been Kavanaugh’s error alone.
I too was curious when I saw references to “Renate Alumnius”. I don’t think it is an error, typo, joke, or mix up. It’s been many years since I studied Latin, but the ending -ius is the masculine nominative declension of the word. It is a legitimate suffix and as with anything Latin, small changes in word endings can change the meaning of the word. The nominative ending makes the word active rather than passive, thus changes the meaning of a subject, alumnus (former student or pupil of), to a verb, alumnius (a former student or pupil acting upon/interacting with) the subject; which in this case, Renate Alumnius.
This is my understanding and I may be wrong. If someone else can provide additional information, I’ll gladly accept it.
Speaking of declension, what about the first word, “renate”? I have seen suggestions that it is supposed to be someone’s name, “Renate” being the German spelling of Renata. But in a Latin context “Renate” could only be the vocative case of Renatus.
ETA the New York Times has identified a candidate Renate, but, again, that is not how the name is declined in Latin. Textual evidence (i.e., the second word) suggests the author flunked Latin in any case.
IIRC, even in Kavanaugh’s testimony yesterday, he admitted that it was a reference to the young woman’s name. He argued that the “alumnus / alumnius” was that they all liked her as a friend, and wanted to honor her. Suuuuuuure.
“Alumnus” (pl. “alumni”, f. “alumna”), of course, usually refers to being a graduate of an institution or a former member of an organization. It sounds very odd for a group of (male) friends to describe themselves as “alumni” of another (female) friend in order to “show affection” for her or to show that she’s “one of them”. Especially if said friend didn’t even know about that “term of affection” at the time.
If, on the other hand, the term was an indirect way for a group of boys to boast that they’d all been sexually intimate in some way (or at least pretended to have been) with a female so-called friend, and were using her name to designate a sort of sexual rite of passage or learning experience that they’d all shared, then the usage makes perfect sense. Disgusting, of course, but perfectly comprehensible in its meaning.
I think you missed a key point in your first link. The -ius inflection turns a noun into an adjective. That does indeed change its meaning, but I’m not sure to exactly what in this case. However, the noun modified is a woman’s name, so the adjective should have been feminine, not masculine: Renate alumnia.
I don’t think this makes sense, since “alumnus” is already adjectival in the sense of “that is nourished, brought up”. Also, not all adjectives and with -ius; compare the related adjective “almus” and note that “almius” is not a word. I may well also be missing some subtle distinction between substantive nouns and adjective nouns, but it would be good to have a better explanation (also of why the word(?) “alumnius” does not occur in the literature).
If the phrase referred to Renate/Renata, why does it appear next to some guy’s picture among a list of his other titles? A simpler explanation is that it is an epithet of him.