Really, that says absolutely nothing about Marco and Ax. That's like saying that since you and I are both gay, we're a thing. Which if anyone is confused, no. We're not.
I never got the Marco/Ax ship at all, really. Yeah, they're the only two main characters that never had a lasting relationship, but both of them seemed pretty exclusively interested in the opposite sex. Of course, I'm not one to question Michael Grant himself on this sort of thing, but even aside from that, when did they ever show any feelings toward each other aside from being friends and partners in the war?
To me, Marco/Rachel would have made a great couple. They had a lot of moments together, especially early in the early parts of the series, and they just had great chemistry.