My hypothesis is that the new free episode is a prototype for a new game, which would explain why it wasn't just a set of levels, but a testing ground for a lot of new interesting mechanics.
The meta-progression through an NG+ cycle, ammo / max HP upgrades, unique end-game fit weapon loadouts through Bloody Shotguns, definitely fit for a modern game design, it's somewhat similar to what Doom Eternal did.
It's also an interesting name considering the expansion is developed by MachineGames. "Quake: Dawn of the Machine" as in dawn of new MachineGames Quake era.
It's especially weird that the new episode is completely free. Why would Microsoft allow MachineGames to work on free episode for so long if not to prototype and advertise the IP for a future purpose?