The advertising thing in the EULA is to "protect for" the possible inclusion of such a feature in the future. It gives you, the user, the chance to opt out (by not buying the game) and allows Frontier to include advertising in the future without changing the T&Cs (which is always a bit hairy). It doesn't mean they have plans to do it, or that they'll ever do it. Just that they can if they want.
In the first instance, Frontier's revenue stream will be from game sales. It's unlikely that everyone who will play this game has already pre-ordered; there are people who won't have heard of it yet, or who are holding off for reviews. So there's a good revenue stream there. Every £40 sale is £40 on the top line as there aren't any publishers taking a cut. Every 100k of sales approximately funds one year of development costs for a team of 100. There are obviously more variables here (such as the company wanting to eventually make a profit and start paying dividends) but they're the rough sort of numbers we're looking at. We don't have much idea of server costs, but I'd be surprised if they came anywhere close to the cost of salaries; Frontier are using cloud hosting, which reduces up-front costs and generally prices scale well with additional load.
Once the base game is out, Frontier will be looking at ways of leveraging income from existing customers (us). That'll be through cosmetic microtransactions in the first instance, but ongoing they'll be looking to get the expansions developed and for sale. They'll also be using the game as a tech demo for the COBRA engine and digital distribution infrastructure, which they'll be looking to license to third parties. This is good for us as gamers as it'll encourage Frontier to make continual improvement to the engine. They'll also continue developing games for publishers, which keeps the company's finances ticking over and provides a buffer for the self-publishing venture.
I wouldn't be against in-game advertising as a source of revenue (assuming it's done well) but I can't imagine Frontier are planning on relying on it, as they'll have little idea of what sort of revenue it might bring, and that's a big risk when you're planning for the future.