Why does "finally intent" represent a shift?
It's important to understand the whole meaning of the sentence here—it's not just the "finally intent" that represents a shift, but also the "For a writer...". The word "for" often indicates some sort of shift. So we can summarize the sentence like this:
Thompson, the writer, has a reputation for being very wordy and difficult to understand. However, now he is finally changing this style, and attempting to make his stories easier to understand.
If we say that he "finally is intent on" something, it means that he was not intent on it before, but now he has changed his mind and now is. So since in the past, he did not care about making his writing easy to understand, now he does care about making his writing clear.