It seems like Luminous Studios is not the only developer proactively hiring people for the development of game for the next generation of consoles. Apparently, according to a report by Gematsu, multiple Japanese developers appear to be doing the exact same thing right now.
A “major games company” based in Koto (in Tokyo) is hiring “localizer/translator/planner” to assist with the development of a game for “next-generation hardware”. We also have a “game developer with [a history of] strong console game development” hiring a planner to work on a game for “next-generation hardware”.
All of this seems to indicate that major Japanese developers, at least, are preparing for the release of the PS5 (and likely the Xbox Scarlett too). Given that current reports peg the PS5 for a 2020 launch, and game development these days takes the better part of a few years, the timeline seems to line up as well.