Basically, I don't know how to reconcile these two opposing viewpoints. If you want to risk spoilers, have a look at Ellen Moody's calendar, linked below (which I've also linked from another message). It incorporates a detailed essay on just this issue. Her two basic points are:

1. There is an Austen family tradition that Elinor and Marianne was an epistolatory novel.

2. The story is constructed on epistolatory principles, namely several concurrent storylines that work well when they take the form of juxtaposed letters. These dramatic contrasts fade when they are incorporated into a single narrative storyline with the subsidiary stories told as flashbacks later in the narrative.

It might also be worth remembering that Lady Susan, her brilliant novel-in-letters, was possibly written in 1794, around the time of Elinor and Marianne.