[TUTORIAL] Creating 2 node Proxmox VE cluster with StorMagic software defined storage

That's right, the license is valid per node sharing storage, not per qDevice (witness).
The witness, however, doesn't need to be locally hosted; it can also be in the cloud and manage multiple storage systems.
 
The witness, however, doesn't need to be locally hosted; it can also be in the cloud and manage multiple storage systems.
This might be true for StorMagic but not for Proxmox corosync qdevice. The qdevice is a Debian box running the qnetd service