> I’m native French speaker and do not understand what you mean with “avant la lettre”. That translates to “before the letter”, what do you mean?

Metaphorical language, of which French is a rich source, often prevents native speakers from understanding. Literal translations are of no avail.

> avant la lettre : before the letter : before the (specified) word or concept existed [0]

[0] https://www.merriam-webster.com/dictionary/avant%20la%20lett...

I think they meant the same as the latin phrase "ante litteram"