From my reading of the scene I gathered that a Quya was onboard, creating an echo of the ground for the chariot to ride on. An invisible road basically, in the same manner the sorcerers are so often seen to "walk in the sky".
I think the chariot and horses are totally mundane and the the embassy involves the envoy Nin'Sarriccas himself, a charioteer/pilot and Quya to provide the sorcerous road.
I dug out some quotes from my copy (Penguin Trade Paperback) for reference, p355-356.
"He saw black horses-a team of four. He saw wheels..."
"The sorcerous chariot rode a low arc over the churned landscape. Sunlight flashed across panels laden with graven images. Sorweel saw three pale faces hanging above of the gilded rail-one of them shouting light."
There's your Quya.
"The hooves of the blacks bit into the denuded turfs, and wings of dust and gravel sprawled about their glossy flanks"
I think this is when they touch down, and transition from riding an echo of the ground, to real ground.
"The centre figure leaned back, pulling hard on the reins."
There's your charioteer.
Hope this helps clear things up!