This is of course, implying anything on netplay isn't already casual.
If you just play netplay, you're (probably) casual. Its harder to say at this point since places probably don't have scenes yet so the statement isn't as sweeping as it could be. Splat Zones isn't more or less competitive than Turf War, its just a different game mode with a little letter tacked to it.
I'm personally getting sick of people thinking Splat Zones is "competitive" (and Turf War is casual in comparison) and that rank has a meaning.