Sadly they're talking both about all new cars and tracks and about putting sweet spinners on your wheels. A customisation graveyard. In a recent Feedback Friday reddit thread, DiRT Rally community manager Lee Williams explained the issues surrounding mods in a Q&A session.
Essentially, it comes down to legalities. They can't allow new cars to be modded in "due to licensing." There is a massive, complex web of who can and can't use certain lookalikes and names for almost all sports and real-world products, from guns to football players. For example, Porsche currently 'belongs' to EA. This also extends to markings, parts and everything else that goes into building a digital racemobile.
As for custom tracks, Lee says that they don't have a program that would allow regular users to make them. Depending on which engine is being used for the game, it's possible dev tools could be used, but they are likely to cost money. Building a track editor would be costly, time-wise, and for a game operating in Early Access probably more effort than it's worth, at least currently.
Lee says they'd really like to support mods, but always hit legal issues when they look into it. He goes into further detail in this post. As a reply says, many mods have used licensed content in the past without any repercussion for them or the developer of the game. It's possible that car companies are more stringent about this sort of thing, or just that Codemasters aren't willing to take the risk.
The full Q&A is worth a look if you're interested in the game. The initial post goes into some detail as to where the game stands "half way in to" Early Access while the comments are filled with answers to various questions, not just mod-related. Based on that estimation, the game should be out proper come November. Official estimate is still before the end of 2015.