[catapart]

"el" begins with a vowel sound, so "an" is the appropriate article.

Is not about the letter, it's about practical pronunciation. "An r before a u, and an m or an f".

[akkad33]

It depends on how you read it. You could say "a large language model".. I think both are right

[CRConrad]

Then the "an" tells us that the author pronounces "LLM" as "Ell-ell-em".