It's part of the official origin story that was published alongside the introduction of the script, so students will learn about it at some point long after they're already fluent readers and writers, and then promptly forget about this bit of trivia. (Do you remember that A is an upside-down ox head?) It probably doesn't help that the original explanation covers Middle Korean for an audience literate in Chinese: https://ko.wikisource.org/wiki/%ED%9B%88%EB%AF%BC%EC%A0%95%E...

Meanwhile, "Korean writing is so easy and logical you can learn it in no time at all" has become a meme to the point where I suspect the number of people who've been exposed to the meme and don't remember a single character might be larger than the number of Koreans who've heard about the tongue shape thing and still remember it.

Also, ㄹ is obviously anatomically impossible for human tongues. It does however closely resemble similar letters in some Brahmic scripts. I'm partial to ʼPhags-pa ꡙ https://en.wikipedia.org/wiki/Origin_of_Hangul#%CA%BCPhags-p...