I'm sure people said the same thing about nuclear energy until it was very, very clearly demonstrated.
It has very little to do with aliens, however.
That's a fallacious comparison: E = mc^2, the discovery of the neutron, and the discovery of uranium's fissile nature told us very clearly how to get nuclear energy (getting nuclear energy _controllably_, however...). Special relativity told us exactly how much power was available. Solid physics.
With ZPE extraction, the physics doesn't give us any way of accessing the energy beyond the Casimir effect, which is pretty restricted. Actually powering anything is in the realms of perpetual motion machines; wikipedia discusses it better than I can here: https://en.wikipedia.org/wiki/Zero-point_energy#Free-energy_...
(Edit: Rutherford did say the idea of nuclear energy was "hogwash", so you're right there...)