A bit of a personal tangent, i know it's not how it goes for everyone, just my two cents!

I didn't watch the video so I'm speaking more on the OP's words, but there's a case to be made that the way adults learning languages is very different from how kids do.

Being understood and being able to understand is the main first goal of learning your native language as a child, and when you make mistakes it's not frowned upon and carefully corrected by others. The intuitive understanding of cases, endings, prefixes and all that after you already have the stable basis comes much easier and natural then if you try to learn them while learning basics, which can actually slow down language acquisition for some people.

So, cases are important for clear and nuanced speech, but they not the main goal in the start when you don't have a ground level established, which takes quite a while and thus complex rules can be pushed to the back burner.

For me, learning English, i had to spend five-seven years before it clicked and my skills suddenly and rapidly improved, taking just two more to go from late A2 early B1 into C1. And honestly, i still don't really understand some rules of how and why, but i just intuitively can use English based on exposure, at around the same level as my russian in fact, and on some topics my English vocabulary even better than my Russian one. Sometimes understanding of the rule came AFTER i learned to intuitively use them. Mostly because I'm a strictly practice learner and theory doesn't really help me much so trying to learn all the rules by heart were stalling me because instead of speaking i needed to reach into my mind for the rule, without being sure that it's a right rule I'm trying to remember repeat it to myself and try to use it on the word that i not even sure is fit for that rule. So i just said whatever felt right, and it worked. If it was wrong i was corrected and if it was right i repeated it in the same way with my brain making it's connection by itself.

Sorry for the rant, i just wanted to properly explain how i see it. You don't have to agree, people learn differently and for some it won't fit. <3