Usually flashbacks are told by a character centrally involved in the action.
Here our narrator Ellen recounts plenty of detailed private events and conversations that she could not have witnessed.
Apparently the book is structured in the same manner, but perhaps it works better? What is the logical explanation for Ellen's omniscient point of view? She's a serial eavesdropper filling the gaps with secondhand information? Seems weak.
In the book, I don't seem to recall that Ellen recounts too many events that she hadn't witnessed or heard about from others, but it's been quite some time since I've read it. If creative license isn't enough of an excuse, then consider her central role in the household. She has known Cathy since birth (interestingly, in the book I seem to recall she was portrayed as close to the same age as Cathy and Heathcliff) So in the book they grow up together, and in the movie Nelly essentially raises them both. Wuthering Heights is not a household that observes hierarchy, so she moves freely in the same realm as Cathy, leaving her free to observe. She is at once mother, sister, best friend, and servant. Furthermore, it is doubtful that anybody in the eighteenth and nineteenth centuries knew what went on in a household better than a servant. They were always present, and witness to everything that went on, and what they didn't witness firsthand was quickly filled in by servant's gossip. This was true in the great houses, and certainly it would have been doubly true in a small house like Wuthering Heights. Lastly, we must consider just how small the world is for our main characters. We have the Earnshaws, the Lintons, their servants, and that's pretty much their entire community. Anybody who's lived in a small town knows that everybody is omniscient about everybody else, and in this desolate, remote county with only two notable families, their servants, and seemingly nobody else around, people probably don't have to poke their noses to far to know what is going behind closed doors.
shareThe book is very carefully-structured, and Emily Bronte tries very hard (and I think succeeds) in letting Nelly Dean find out information that she wasn't privy to: many conversations between herself and Heathcliff and herself and Cathy are reported; Isabella writes Nelly a long letter after her escape from Wuthering Heights. There are other written and overheard or spoken incidents that Nelly remembers or still has access to. So that aspect of getting the story is nicely taken care of - and because we the readers never get the story straight from the horse, Emily Bronte is implicitly inviting us to question all these sources!
Anne Bronte, in _Tenant of Wildfell Hall_ (much under-rated still, though gaining an audience), has a simpler but similar structure. Unfortunately, because of copyright nastiness, there were and are several corrupted editions of _Tenant_ out there, which chop off the introduction and other portions of the novel, making hash out of the structure. Don't buy any edition that starts with "You must go back with me..." You need to find an edition that starts with a letter to Halford. This letter sets up the reason for Gilbert relating the story, and the sources he's using. Also helps with the general tone of the novel.