The difference is how it is percepted by the masses. MS will obviously want to distance itself so far away from Windows Phone that they need to market this new device correctly immediately from the start. They also need to ensure that more than 5 markets are targeted as well and just make it available in as many countries as possible.
To answer your question, most people will still see Surface with phone capabilities as an extension of Windows Phone (even though the Surface brand in its own right is entirely separate and doing much better) and that will lead to the app gap and all the negativity surrounding Windows Phone being heaped on this new device so it may die before it even gets a chance.
Essentially MS need to handle this entire thing with kid gloves and ensure they don't make the same mistakes again. But, unfortunately, they most likely will if history is anything to go by. Maybe by the time of Surface Mobile relaunch 4 (where 1 - 3 versions failed) they may get it right but by then it'll be too late for anyone to care.
You're right, if people doesn't see it as a mobile phone, no Windows smartphones are going to be used. But, then, why is Microsoft developing this device? If they just want to sell a new Surface, they can do it without mobile functions.
As you say, the time is important, the more they wait to get right with the mobile device, the more late for anyone to care. Maybe, if people who is waiting this device as a new Smartphone to use with Windows Mobile (this website is full of people doing it) start to use it, and it has success, the image of Smartphone start to spread itself slowly.
When first smartphones appeared, only few people started to using them, but the effect sensed by the others spread the smartphones market. And I think that the effect of someone that can have a full integration with his mobile device and PC can be a huge candy.