Why did you merge it? I intentionally posted it there to receive needed answer from @unman on how to make it disposable.
The command certainly accepts disposable to create a disposable sys-net instead; it just happened that I don’t know how, but it’s certainly trivial for @unman to answer. You can indeed verify that by looking at /srv/formulas/base/virtual-machines-formula/qvm/sys-net.sls, the line I mentioned above.
Also, see the post @solene just created. It has the way of creating it even without Salt, although I see no reason to follow the guide and not utilize qubesctl state.apply qvm.sys-net.