S
SammyInnit
The old gully could have dried out and started working as a vent.
I'd say it's down to the fact he's got a great massive run off (presuming that's that the photo was for running through his conservatory), a second trap which isn't needed, both washers draining after the second trap and neither one not having a high loop and no AAV.
There's no way on earth that was done by a plumber.