Do you happen to know what database they use to determine frequency and maximum and minimum uses? Just dictionaries? If so, whose and what types, for ex.? All books everywhere? Online books?

I'm pretty sure they use the classical texts which they have digitized and are available on the same site. Classical philologists were amongst the first in the humanities to use computers. There's a bunch of concordances from the '60s of Latin and Greek authors. David Woodley Packard, the son of one of the founders of Hewlett-Packard, started the Packard Humanities Institute which first digitized the extant corpus of Latin and Greek classical texts.

Ceci n'est pas un seing.