Knowing Apple and their ways, I can almost guarantee that this won’t be even come close to being capable of game development on the scale of Unreal Engine 4, likely just some marketing thing, not to mention that not even any other tablets are capable of game development, as none of them are powerful enough.
Edit: It’s looking like the price is going to be from $800 - $1100 for their new product, and boasts to be more powerful that most popular laptops, but we all know how much Apple bumps up their prices after production costs, meaning that the cost to make these will be pretty low, meaning that they either invented some new awesome way to get higher performance for much much cheaper, or they are just trying to appeal to those looking for something better. Yes, I am hating on Apple, I have ever since I got my first Apple product, but I still stand by what I said above.The way I see it, anything you can get from Apple, you can get the same performance from someone else for 1/2 the price.
Edit 2: After doing some more reading, it’s looking like this is going to run iOS 9, and all hatred aside, iOS has many many restrictions that prevent all sorts of apps from running the way they do, or making it onto the app store.