Within is an app developer whose apps compete with apps owned by Meta, run on hardware owned by Meta, and are sold in an app store owned by Meta. Why does this confuse you?
There is no viable anticompetitive case for VR as it is a nascent industry which barely exists. Furthermore, Meta is far from the only player in the VR space today - Valve, Pico, HTC, Samsung, and Microsoft amongst others all play a role in the VR ecosystem.