"negative pressure zones" is meaningless nonsense. They are ventilated along with the ebays into the underfloor area by the outflow valve exactly like the Ejets 15 years earlier.
Its not meaningless its pretty standard environmental engineering in buildings. And it works.
Proof of the puddings is if someone takes a dump in the forward bog the flight deck doesn't know about it. Ejets you get the full flavour and sound effects in the cockpit.