That makes sense, except that would lead to double spaces in certain places. They'd have to represent many different things, which we have no way of knowing until we figure out something else.
The letters may well be nulls, simply there to confuse us.
I think double spaces are okay. After all, the main point is to get the recipient to be able to understand the message. That means you have to have separators for words so they don't misinterpret and blend them together, and you need separators for sentences.
Good catch Tylui with the letters ascending regularly. That suggests the letters are just spacers that we should ignore, but I think that's unlikely: why would he have exactly all 26 alphabet letters present once each?
Also a bit of meta-decoding: if it were based on a decoding tool that we don't have, it would be too difficult and he would never have used it. A good example I've used before is a common book that both the sender and receiver have access to. Your code is a series of page numbers. The translation is each page's first letter. You use multiple pages with the same letter to throw off frequency-based translation. Pages divisible by 11 are used strictly for punctuation and those divisible by 13 for spaces, pages divisible by 9 are used for numbers, ascending. Unless you know and have the book you're pretty much screwed. And if both sender and receiver have many books, all without marks in them, how in the world will you know?