Note: I already reviewed many topics here regarding the use of わけ, but I still struggle to properly translate the sentence below. It's from the introduction to the book 日本語文法ハンドブック (a book for teachers of beginning Japanese grammar).
Translating literally produces something like
There's no need to explain it, but for real [actual-use] Japanese, it's not the case that distinctions between early and middle/advanced Japanese grammar exist, in other words early Japanese grammar is setting an expedient foundation [for teaching someone to learn].
Which sounds like utter gibberish to me. The stumbling block is the phrase "初級と中・上級との区別が存在しているわけではなく". I would think "存在していないわけではなく" makes more sense ("it's not the case that distinctions between different levels of grammar don't [already] exist"), but that's not what's used here.
What is a correct translation of the sentence above, especially in regards to the ~わけ phrase?