I think there's also the question of whether a word adopted from Danish by an international standards body in the 20th century automatically counts as English.
Use “femto-“ in a sentence.
> Use “femto-“ in a sentence.
There you go.
Even if we discard obligatory quotes, there's still the hyphen as a part of stand-alone version of this "word". So when I use "femto-" by itself in a sentence, it's at best 6 characters long.
You can quote any nonsense in this sentence, so it is an incorrect proof of a word’s existence.