Agreed that the challenge exists for all languages, though I will say I strongly disagree with the assessment that this difficulty in parsing is a significant cause for the substantial difference in vocabulary acquisition that Japanese requires.
Because first I believe the researchers do their best to keep the counts between languages as analagous as possible
Second, even with languages like Korean that have similar grammar, there is a massive difference.
And finally, because the number of additional words you could gain from separating out stems and conjugations is finite: once you have て、い、る、ない counted up as different words in the most spread out way possible, no plain form of the simple present or present progressive will add a new word to any verb stem. So maybe this could inflate the figure by a couple hundred at most, but it’s not enough to explain why in Korean, 5000 words gets you just shy of 90% vocabulary coverage whereas in Japanese, 10000 words gets you just over 90% vocabulary coverage.
So yeah, even though I cringed when the author brought this reason up, I learned something new out of it xD