One of the more helpful inputs I garnered from a university group read of an Austen novel was Jane's use of "free indirect speech" (also discourse, style, or narration). I'm not an authority by any stretch of the imagination, but the professor explained that Austen is the first English novelist to employ this shifty style of narration, whereby the omniscient narrator snakes in and out of subjective views of individual characters amidst presenting the more traditional, objective and authoritative third person accounts typical of ON's, without necessarily signaling the shift overtly.

It's been eye-opening for me to read Austen's novels with this in mind. As I understand it (an admittedly highly qualified limitation), this style is a sophisticated form of narration that calls into question how exactly Austen is employing the narrative voice in a particular instance--relating something we are to take as "bible truth," presenting solely one character's purely subjective stance (often in the style of the character themself), or some point in the middle of the continuum formed by these poles. The approach allows Austen to relate the interior life of varying character without being bound to any one's point of view through the course of the novel. I have found that she uses it to varying effect with respect to a particular character in the course of a single novel. This has kept me on my toes as I consider the range of possibilities that input from the narrator may be serving, highlighting how innovative, subtle, and even slippery, is Jane Austen's style.

As Barbara points out, here it does seem that the narrator is "speaking" Willoughby's POV. There is no reason to believe it is not an "accurate" description of the Dashwood sisters, and provides some insight into Willoughby's additional motive for returning to the cottage to see after Marianne, as "Of their personal charms he had not required a second interview to be convinced."