The author definitely benefits from French and English being related languages. Remembering that "interesting" is "intéressant" takes a lot less effort than remembering it is "おもしろい".
I studied Japanese full time for a year and would probably average something like 20 word a day but even that was pushing it. I advise people to aim for 3-5 words a day and be consistent. Adding huge amounts of words very quickly and then giving up on reviewing them is all too common.
Author here. You're absolutely correct. Make that deck 5000 non-cognate words and 50 words a day quickly becomes unsustainable. Not to mention the alphabet(s) required for a language like Japanese. French is a language I think any decently well-read person (so as to recognize many of the cognates) can learn vocabulary in surprisingly quickly, considering how much of English vocabulary is derived from French.
That's a lot! I could never do that... I'm learning Japanese, and I could never imagine myself doing more than 10 words a day.