Making Korrasami canon was a hail mary. They knew the show was shit, they knew most fans hated book 4 (including everyone who was won back by book 3), and they knew shippers were one of the only parts of the fanbase they had left.
The fact that there was absolutely no buildup to it, that final clip lasts like 20 seconds, was unvoiced, and doesn't show anything but them holding hands really makes me think it was tacked on at the last second and wasn't included in the original script. Yeah, the final shot of Aang and Katara kissing wasn't voiced either, but they had kissed several times before that point. Korra and Asami holding hands just felt like a total non sequitur.