Well, if the mod URD was making wasn't very good I doubt Kunos would have included it. But URD is known for their excellent quality so it was a no-brainer. I'm sure a license is still required in order for it to be added as official content, and the manufacturer probably wouldn't sign off on it if the quality wasn't very good. The Cobra was also added, not sure if Kunos already planned to add that car or if it was just due to its good quality, but it started its life as a high quality mod.
Same with Zandvoort: "The Dutch Zandvoort track is especially remarkable as it is the first track in Assetto Corsa not to be laser-scanned. Unlike the other tracks, Zandvoort hasn’t been produced 100% in-house but is based on a modding track that has been brought up to AC standards."
http://www.virtualr.net/assetto-corsa-first-zandvoort-footage
I wasn't saying every high quality mod will be included, but if a mod is going to be included it will likely need to be high quality (and probably something they were already looking to include). I'd say it's pretty unlikely that any/many mods will end up as official content, but there is precedent for it happening.
I was just trying to point out, mods which don't become official isn't a sign the mod isn't high enough quality. I should have said being a high quality mod doesn't mean it will ever be official content. But yes it was pretty much a given that URD would make a high quality car. I've read on the forums that the muira is the same as the mod car.