I don't know... even if you put it in a seinen magazine, the art style, characterization, focus, and relationship treatment all look "shoujo" to me. Maybe it's a shoujo author trying to write seinen, but unable to shake the shoujo. Maybe it has a lot of cross-appeal. Maybe the later chapters will be more "seinen." I don't think it was much of a twist, after being foreshadowed to that extent, but maybe it'll be much different later on. For now, it feels much more like a shoujo series than a seinen one.
I don't mind that. I'm just observing that the tag is, at the moment, more misleading than accurate.