Its not just "people" saying its canon, The writers themselves even say that Korra and Asami are canon. The only reason they didn't represent it more was the fear that representing the LGBT+ community further would get their show taken off the air, otherwise they would have. Not to mention nothing in the show was "planned", The only thing planned was Korra's story arch, everything else was improvised.
One last thing, It does not need to be canonically written into the show for it to represent the show. To name a few Raava and Vaatu aren't human, Korra isn't male and I don't recall Jinora ever being interested in Mako, all three made it into this list and yet are NOT canon. It doesn't "NEED" to be canon to represent show or even for fans to want to see more of.