My frustration has very little to do with what's canon and a lot more to do with the way fandom shippers feel like the lack of explicit confirmation can be used as a bludgeon against people expressing discomfort with the ship when fandom is a media space rampant with romance-centrism and when the coding is as explicit as it is. And then try to throw out "I accept he's "canon" ace" as a way of shielding themselves from criticism despite that not actually being anymore canon than anything else. Also, frankly, I think people would be entitled to get mad if they walked back from this coding. You talk about fighting for representation, part of doing that is voicing criticism when representation falls flat. (And walking back the coding would be falling flat even if that wasn't the intention.) I don't actually think Riz is canon ace or canon aro. Because I do believe in death of the author and that the ability of the audience to interpret art is very important. And I agree that heavy coding alone isn't enough to make something inarguably canon. The problem I have is the way that so many fans go out of the way to dodge Riz's potential as representation and refuse to support aro fans who see themselves in Riz. Mostly, as far as I can see, for the sake of continuing to ship a pretty bland ship. And a little bit to protect the creators from some potentially deserved criticism. (Also, having not watched Voltron, I can't speak to the accuracy of the Klance comparison. But generally ship happy fandom exaggerates queer coding on everything except ace and aro coding. So I'm inclined to suspect it's not entirely apt)
There's is no problem with aro people relating to Riz and seeing him as Aromantic, but if you don't like when people ship Riz with people, just block them. Other people shouldn't have to stop having different interpretations because they make you uncomfortable.
Maybe people are dodging representation for the sake of a ship! But if that's your issue, I think you're talking to the wrong person. I'm not saying he isn't, and I'm not trying to ignore the coding. I'm simply saying that it isn't canon. That doesn't mean that the characteristics that make people believe he's aromantic aren't there. He still has those traits no matter what his sexuality is.
I have said over and over that there is a lot of coding, and it's obvious why some people believe that he's aromantic, but stating that he isn't canonically Aromantic isn't an attack on anyone, it's a fact.
Also, please don't get mad at Murph if he doesn't make Riz canonly Aromantic. Coding doesn't equal representation, and it's his character.