It’s an acronym, tho. From the Wiktionary page for ‘acronym’:

The broader sense of acronym inclusive of initialisms (as TNT) is sometimes proscribed[3] but was the term’s original meaning and remains its more common meaning.

So it seems like a bunch of prescriptivists just made up this distinction, included it in some style guides, and then tried to proscribe the other meaning or something.

Don’t tell me how to pronounce TLA.

https://en.wikipedia.org/wiki/Acronym

In English, the word is used in two ways. In the narrow sense, an acronym is a sequence of letters (representing the initial letters of words in a phrase) when pronounced together as a single word, like NASA, NATO, or laser. In the broad sense, the term includes this kind of sequence when pronounced letter by letter (such as GDP or USA). Sources that differentiate the two often call the former acronyms and the latter initialisms[1][2][3] or alphabetisms. However, acronym is popularly used to refer to either concept,[4] and both senses of the term are attributed as far back as the 1940s.[5]

Because we have so many ten letter acronyms and exponentially more twelve, thirteen and thirty letter ones, that its simply too confusing for the advanced modern mind

Rather, two letter acronyms are less common and three letter acronyms are highly favoured in this language of ours

That’s why YSK.

But nothing beats hot WAFLS in the morning!