Basically here's my rule of thumb: any linguistic claim coming from a mainstream source which seems to imply the Sapir–Whorf hypothesis[1] or other kind of objective criteria to compare languages is a huge red flag and extremely likely to be entirely bullshit. Out of those claims the "language X is more precise/able to express more nuance than Y" is probably the reddest flag of them all.
I think generalities about languages ("French like to say no") fall in the same category as horoscopes: if they're vague and general enough you'll probably recognize yourself in them to some extent. People like to feel special, they like to feel special for speaking a given language so they'll gladly accept any claim that sets them apart.
>If true (I suspect a methodology shenanigan there) I really doubt this is a factor. Very few French or English speaker will have a mastery of 70,000 words, to say nothing of 500,000.
Yeah, this claim alone shows that the author really stayed at surface level. I suspect the main reason for these numbers is that English, traditionally less prescriptive than French, considers basically any word once used in an English text to be an English word while French curates more heavily. The average English speaker definitely doesn't draw from a reservoir of 500k words, probably closer to a tenth of that. Maybe somebody could link this website to the author: http://testyourvocab.com/