I would have to say no, they are not a couple. They were not a couple at any point in the show, and every incident that hints towards them being more than friends at any point in the show needs far more justification to say they're romantically tied rather then being just friends.
I don't say this because I don't like the idea of them being a couple. I actually really like the idea of them being together. I say this because the "hints" and "evidence" is inadequate to prove they were more than just friends. Every other couple or crush on the show is talked about and stated in very clear terms. Think about how every other character talks and reacts around people they like. Clear, obvious, no subtlety. With Korra and Assami, you need to pick apart scenes and do some mental gymnastics to "prove" it.
I feel like the creators saying they were (or became a couple after the show ended) is a cop-out. They claimed to have an LGBT couple, yet did nothing within the show to... show, it.
So I say again, they are not a couple, and anything someone could say to show otherwise could just as easily be due to the close bond they shared after being kidnapped together.