In this language learning method there is a goal of 5000 "words".
I think it is better to count "lemmas" instead of "words".
learning 5000 lemmas (and all forms of these lemmas) represents a good knowledge of a language but 5000 "words" is not so.
I am learning Russian. In Russian the verbs conjugate; every noun has (up to) 24 different forms as well as adjectives an adverbs.
in this case learning 5000 word means less than 500 lemmas and even less.
Many other languages have similar situations with some differences based on their grammars.
I think it is better to make the vocabulary goal for such languages just based on lemmas .