I have to admit that it's so frustrating to me that people are doing this "He's canon ace but not canon aro" thing. It feels like people wanting to have it both ways at the cost aro people. If you're going to say only what's explicit in the text matters, then be honest with proper embrace of Death of the Author and disregard what Brennan said outside the context of the text. If you want to invest meaning in authorial intent, than be honest about the authorial intent with Riz. It's hard enough being aro in fandom as it is without people playing both sides of the fence on this
I'm sorry that there is a lack of aromantic representation, but you can't decide what the authorial intent was for the author. He could be aro! There's a lot of subtext and ways to read the story like that, but it's just as easy to read it as him being demiromantic or gray romantic or even just a late bloomer. Because of this, I can't confirm the authors intent unless he states it. I don't personally have an opinion on it, I just like Riz, and I'll like his character no matter what his sexuality is. I'm not going to disregard a statement from a cast member, especially when I doubt Brennan would have said without Murph's permission. That, to me at least, is confirmation of authorial intent.
I can understand why you're frustrated, but there aren't (or at least shouldn't be) sides. I'm simply saying that Riz is not canonically Aromantic, which is true. However, this does not mean that Aromantic people are somehow wrong in saying that they believe he is! Just like many other things, it is up for the viewer to decide currently.
If Murph comes out and says Riz is Aromantic, that's it. He's Aromantic. But until then, it's up for interpretation. You aren't wrong for saying he's Aromantic, but neither is someone saying he's not Aromantic.
I am so glad that Riz resonates with you, but others don't have your same experience and might relate to him in different ways, and their experience and interpretations aren't less valid than yours.
The world isn't black and white. There are so many nuisances that can be beautiful if you allow yourself to see them.
I'm not on the fence. I know exactly where I stand. Riz is canonically asexual and he has no confirmed romantic interests, although he has been shown to be less interested than his peers. This could mean many, many different things, but saying that thinking Riz is asexual but not aromantic is being on the fence is discounting real people who are asexual and not aromantic.
It's not all or nothing. I'm not out to get you or hurt anyone. It just frustrates me when people state their interpretations of media as facts and get upset when other people have different interpretations. Your interpretation isn't more valid just because it's more diverse. That would be like me reading a book where someone was never stated to have a race, but I decided that their character would make more sense if they were black, so now they're canonically black, even though I don't know what the author imagined them like. Just because it's adding diversity doesn't make it more morally correct. It doesn't make it more incorrect either. It just makes it your interpretation.
I know you're probably sick of hearing this, but it's up for interpretation, and you can't fault people for having a different interpretation of a character.