It wasn't, it's the product of a few logical inferences.
The CM system in general needs to be able to produce basically any physically feasible track layout in order to deal with the GPS function. Presumably there will be basic limits on elevation, corner radius and things like the track crossing over itself, but you get the idea. The CM needs to be reasonably compatible with a very wide range of track layouts.
Given that the system has to be capable of doing this from GPS data, the logical thing would be to allow the player full access to "manually" create any track that the GPS function could create. I'm well aware that PD doesn't always do things logically, but I see no reason why the manual CM mode would be limited any more than the GPS mode. It may well end up being, but I think it's a fair assumption that they're both capable of the same things, given that they're essentially different input modes of the same underlying system. (Please Polyphony, do not build TWO course makers where one would do.)
Honestly, I expect there will be limitations. I don't think you'll be able to recreate Suzuka or Top Gear TT with their crossovers. I don't think you'll be able to recreate say, Cape Ring with it's loop and ludicrous jump. I seriously doubt you'll be able to make VRally 2 style super launch ramps. I seriously hope we can't do Trackmania style hypertracks. But anything that you might find on a public road should be fair game*.
And if you can do it with GPS, then you should absolutely be able to do it manually. Frankly, if you can't do it manually then people will force it. If you've ever played any of the GPS augmented reality games like Ingress, then you'll know that you can spoof your GPS sensor in your phone. If Polyphony has a GPS app I suspect I could spoof my way into a track around Antarctica right now, although it might be pretty time consuming depending on which tools work and which don't.
If stuff like this works, then it's a piece of cake.
Polyphony may end up limiting the functionality of the manual editor, but I think that they'd be fools to do so. The idea of it being a free editor is a great one, although I admit to also liking the idea of having set environments to build around as well. Total freedom is great, but building within limitations can be fun too. I just don't see why they wouldn't provide the total freedom option when the software has to be capable of it anyway (given what they've told us so far).
*Although as usual, the caveat of logical interpretations and expectations not applying to Polyphony Digital is in full force.