Well, you have to imagine that some of these tracks have been written with the context of the story in mind, or that segments of the story will be directed with the pace the song in mind.
It reminds me of this scene from Attack on Titan (which he also composed), where Mikasa wants to die, and there's a slow build up with the music for well over a minute, but in context to the scene, it made it feel more emotional when the track picked up pace as she remembered Eren's words and changed her mind.
I wouldn't mind if it was set up like that, but there's a difference. First, that was a piece, not a song. The way they're structured are different, and what they are building up to is different. Second, that peice isn't the opening peice. They made that song out like it was the theme song to the whole game, like Sanctuary or Simple and Clean. That's a completely different setting for a song than that scene.
They never made the song out to be the theme. The reveal trailer's composition was obviously the theme to Xenoblade Chronicles X. Followed up by the composition being the first music available to listen to on the website.
Thinking the song recently posted on the website was the theme was pure speculation on your part.