Except they DIDN'T throw it in at the end. All throughout book 3/4, we see Korra and Asami form bonds of intial friendship(Started off in Book 2, IIRC), and it grows over time. Off of the top of my head:
Asami being the one to take care of Korra while she's talking to Zaheer
Asami being the one who looked after Korra when Korra was recovering
Korra writing and keeping in contact ONLY with Asami. Not Tenzin, or Mako, or Bolin, or anyone besides Asami. That's beyond friendship, that's true trust, and Asami was willing to keep that secret up.
It's in the series, you just need to look for it. What WOULD have been fanservice would have been Makorra, as we saw them constantly sink that ship over and over again.
And all of that is nothing that freinds won't do for each other.
Their was nothing to suggest more.
Kind of felt like a last minute hype job.