> The fact that "100 most commonly used, make up 50% of written words" (a dubious statistic, source?) is irrelevant.

See https://en.wikipedia.org/wiki/Most_common_words_in_English for a claim. It is what teachers use as a guideline. It's the first 100 words they teach children to write.

> the idea is you use a mnemonic generator to pick the words.

I know you are supposed to use a generator to pick the words, that is how BIP39 for bitcoin works. But average Joe is not going to do that. He will select "I went to highschool in 1992". Authentication is a hard problem, and unless you force a reasonable scheme, it will be weak.