The guy answering the questions is the lead writer.
If he "guesses" things, that means that if FS3 was made, that's the direction he'd take it in, barring any changes to the plot for gameplay purposes or whatnot. If he has the power to act upon his "guesses" by writing the real thing, they're not ****ing guesses. You don't "guess" at where you want to take your story, because if you're the author, you are the god of your story's universe - you dictate everything, your guesses are truth.
There is no full story outline with every single major event, every mission laid out for a game that was never made, a story never written. There are vague ideas. He gave us all of that. That's more than we could ask for, you shouldn't expect any more.
Yes, just from FS2 there is ambiguity, and that is a good thing, but this information doesn't bind you to anything - you can still write your stupid story about the Shivans blowing up Capella to harvest gasses from a nebula and forming a Terran-Shivan alliance with Bosch and fighting a Shivan Civil war - these statements don't change the validity of even that ridiculous story. All this does is provide insight to what FS3 may have been, had it been made...yay for self jab