I would not call this gatekeeping, I would call this a simple (albeit loud) critique, and maybe a reality check. Gatekeeping is when someone new is not allowed to enter a certain space, but people here are simply pointing out that this article is not the expert opinion of somebody that know how to teach Japanese verb conjugation, and that tutorials like these, while not harmful, are not super useful either.

Learning Japanese is fun, and I encourage the author, and the readers of this post to continue learning. But my advice to new learners is to just get a textbook (or some structured material; but please, just get a textbook unless you are one of the minority cases where you can’t for the life of you follow a textbook) and learn the conjugations little by little as they advance through their textbook (or their alternative structured material) one chapter at a time and consume more and more advanced Japanese language material over the course of dozens of months.

For the people who do that, I promise you, if you study for an hour or two a day, just following some structured material aimed towards beginners (preferably a textbook) and engage with the language as frequently, you will intuitively understand and be able to produce the mostly correct (i.e. with diminishing errors) by 3-6 months.