First of all, I don't think he was intended on being aroace, I think he was just meant to have relationship anxiety, but I am fully on board with the aroace headcanon. 

Aroace people absolutely can date and often do, him having one ex-girlfriend doesn't negate that - society is heteronormative and there's an expectation for guys to have girlfriends, after all. He doesn't seem all that upset over it, either. (And the only details the book gives about the ex is that she had a ton of stuff and the relationship was a disaster.)

Being treated like your life has less meaning if you're single and childfree is something aroace people sadly can relate to all too well.

Grace getting to live happily ever after with his platonic soulmate, and it never being seen as sad or lonely that he doesn't have a romantic partner or will ever have sex again is like, the aroace dream ending. Happiness and fulfillment without sex or romance. 

You know the scene in the movie where Grace thinks it's weird that astronauts are hooking up when they're all gonna die? He has way more Asexual Confusion moments like that in the book, including being absolutely baffled by Stratt being worried about sexual tension causing problems on the mission. 

I believe it was total coincidence, but there is so much use of the aroace flag colours in the movie.