Mr Alba used some sort of spongy material as an underlay but he laid the wood on top of a hardboard type surface and not floor boards.
No glue as the planks need to have some give when folk are walking over them.
As an aside, I've heard stories that if the wooden flooring is underneath the skirting boards some insurance companies can be funny on meeting claims, something to do with their outlook on fixtures and fittings, but I don't know if that's an urban myth.