Without wishing to claim any expertise whatsoever in the dating of Chinese ceramics, I am not at all convinced by the dating of this figure as Qin.
(1) As Thomas Chen said, the saddle is not at all like the Qin terracotta examples, or even like those Han examples that I have seen. The arched pommel and cantle rather resemble 4th-century or later examples.
(2) The “armour” at the front of the horse doesn’t seem to cover the neck, but to be an apron covering only the chest, and slung quite low. This reminds me of the figurines from the Jin tomb of AD 302 that Dien uses as evidence for “mounting stirrups” – their horses have similar chest “armour”, though unlike this one they have none on the horse’s rump. It makes me suspect that this may be some sort of transitional stage between the 302 examples and the well-known “Northern Wei” style of horse-armour.
(3) The rear portion of the harness, a double crupper-strap and several pendant straps hanging down from the rear of the saddle, remind me of examples on the Wei or Jin tomb-paintings from Jiayuguan.
(4) The same site has a couple of other mounted figures dated as Qin, neither of which look convincingly Qin to me – see 1681/17/2 and 1681/17/1 on the Qin/Han ceramics list.
I suppose the dealer’s informant couldn’t have meant a
Former Qin tomb, could he?

And it doesn’t really affect the date, but since the rider has no armour and apparently no weapon – though we can’t really tell what he was holding – I’m vaguely wondering if it is armour on the horse at all, or just some sort of decorative covering.