When the comment says "understood", read it as "fluently understood". And again, "begging the question" has nothing to do with grammar, so there's no reason to sass by making grammatical errors.

Begging (for) the question is a correct literal usage of those words, under both prescriptivist and descriptivist definitions of the words. You're allowed to use the same sequence of words as an idiom in other ways. I can talk about a hot potato without metaphor, and I can talk about begging a question when not describing a fallacy.