The director's commentary to the film goes a little bit more in depth on this.
First, in the scene immediately prior to this, when Legolas turns away from Tauriel, Philippa Boyens implies that his "I cannot go back" line was about Tauriel.
Philippa Boyens: In this look, I think what you see, in this look of Legolas he knows that he can't go back, that he has lost Tauriel forever.
A bit later, when discussing the farewell conversation of Thranduil and Legolas, they discuss Legolas leaving, and say that he then meets up with Aragorn.
Peter Jackson: This is kind of interesting to shoot this, because three movies shooting them back-to-back and we were filming all of these scenes in the last one or two weeks of the shoot, all the farewells, all the final scenes of the movie were pretty much being shot at the end. And so for us it had a whole other resonance, that it was our farewell to Middle-earth and our farewell to these characters and they were saying goodbye--
Philippa Boyens: But we were also carrying the story on in this scene. Because he's sending him off to meet an important character.
Peter Jackson: Which is pretty much what happens in the book, isn't it? That Legolas, he unites with Aragorn between here and Fellowship of the Ring, isn't it?
Philippa Boyens: Yeah, yeah, you know that they kind of know each other because they were involved in the Gollum story.
Peter Jackson: So how does it work?
Philippa Boyens: Well Aragorn really is the one who tracks Gollum and and finds him eventually in the dead marshes. And he's taken to the elves and because of the kindness of the elves -- and legolas is one of his keepers in mirkwood -- and through their kindness he actually manages to escape... Or has he escaped? Or was he let loose?
Peter Jackson: So that's like a seventh Tolkien movie that you've just heard, only we're not --
Philippa Boyens: Yeah, Good luck, Pete
Peter Jackson: no, it's not a film we're going to be making it's a film that we'll never make but you've just heard a little measured version of it from Philippa Boyens
Philippa Boyens: The hunting of gollum, through the wild
Peter Jackson: The Hunting of Gollum
Philippa Boyens: and a few other things that go on which are really interesting. Saruman's search for the ring is also interesting.
Peter Jackson: It's all part of a fictitious film 3.5
Philippa Boyens: But what that was, it was a little homage that we wanted to do and because we'd always wished we could have had Aragorn in these films.
Philippa Boyens: We dearly dearly wanted to be able to have Vigo back in these films
Peter Jackson: I know, we did actually try to figure out a way to get a cameo, even if it was a cameo for Aragorn and actually for Arwen too. We tried to have Liv Tyler in the film. Because we wanted to just try to make that connection but we couldn't think of a way of doing it without making it too geeky because we didn't want it to be less than what it could have been otherwise. It's no point of doing it.
Philippa Boyens: No, the only thing you could have done was this, it was so slight and it would have left the audience just wanting more and so it wasn't worth it, it wasn't. I mean there was a real function for Legolas in these stories, there was a real function for Elrond in these stories, there was a real function for Gollum, the characters that turn up in these stories from The Lord of the Rings trilogy are ones that have
Peter Jackson: but also I think that the story of Aragorn is something to look forward to now in the next movie because it is the very next film.
Philippa Boyens: Yeah