Even if you're right about that, it still doesn't make the word Latin. Latin speakers wouldn't say it as one word, and there would also be a verb in a Latin sentence - "de fenestra" by itself is nonsense.

(I had the displeasure of studying Latin in school)

> Latin speakers wouldn't say it as one word

Note that all Latin speakers in that time period spoke Latin as their second (third...) language, and it was pretty common to see influence of other (mother) languages onto the used Latin. It wouldn't be surprising if e.g. a German native speaker (where such word concoctions are common place) coined such a Latin word.

It would also be unsurprising to see a native speaker of classical Latin coin such a Latin word. defenestro and defenestratio are perfectly compatible with the normal methods of word formation in classical Latin, which very rarely forms compound nouns, but which forms compounds of verbs with prepositional prefixes all the time. (Just in that last sentence, you can see the ghostly remains of perfectus [thoroughly-done], compono [with-put], praepositio [before-putting], and praefixus [before-fastened], all impeccably classical. You can also see compatior [with-endure], which does not seem to have existed in classical Latin, but is obviously derived in exactly the same way as the others.)

Here ( https://www.perseus.tufts.edu/hopper/morph?l=defatigatio&la=... ) is a dictionary entry citing defatigatio to a speech given by Cicero.