I think for a very specific audience the article is useful: an English speaker who is in the first month of learning Japanese and is having trouble understanding the basics of Japanese verb conjugation.
If you are not in that target audience, the article is not that useful. If are starting to learn Japanese, you would not start by reading this article. And once you are past your first month of learning Japanese, you have internalized how this basic part of Japanese verb conjugation works and thus the article seems hyper focused a tiny part of learning Japanese. So it’s predictable that people would point out the limited scope of the article.
I think “Aeron Buchanan's Japanese Verb Chart” offers a more complete overview of Japanese verb conjugation in a more concise form. It expects you to know how to read hiragana, which is reasonable because it’s one of the first things you learn when studying Japanese.
Intuitively I suspect that you severely underestimate how many people give up on the language somewhere in the middle of the process of learning this "basic part" in the traditional way. My aim was to show to this group that you can actually understand the entire mechanic in one evening with zero prerequisites.