The witness is the entity behind the pyramid ships, so there must be a similar entity such as the gardener inside the traveler, the only way there would possibly be a city inside it is if it let the Ecumene inside. However even though the BoS are not reliable sources, you would think they might have mentioned that, plus if it did that for the Ecumine why not the Eliksni or humanity during the collapse, or when it was going into savathuns throne world, it would have made a lot of sense then. If it was possible for the traveler to do it I’m sure it would have but since there is no record of it happening and that is has not done it for us or the Eliksni during the whirlwind it is probably only housing it’s paracuasual entity (probably the gardener)