I believe this is a matter of perception since there is no clear definition between Cube/ACube.
Both artists are close to each other, in terms of relationship and physical proximity of both offices. Therefore, it is understandable that many love to relate both Cube/ACube artists together.
On a personal viewpoint, I like A Pink more than just a casual stan, but less than a 'stan' stan. It's just a matter of taste - nothing to do if they're under Cube or not ^^
In my opinion, both perceptions are acceptable. Don't have to judge others for 'including/excluding A Pink', because really, there's nothing to fight about. Good relationships (among artists and among fans) are all that matters.