It doesn't prevent improvisation, but I think Matt's gamebook approach may discourage improvisation because it encourages the GM to consider player activity as taking place only within the pre-scripted game matrix. The risk is that if the GM has put (considerable) effort into detailing 3 options, but the players go 'off script' and choose option 4, then the GM may be less able to react than if he had simply created a situation with the intent that the players address it however they wished.
I see this a fair bit with 4e DMing advice from eg Chris Perkins or the 'ID DM' blog - lots of advice on steering players back 'on script' if they err from the pre-charted course. I think Matt was the third GM to post something similar in the past 2-3 weeks; plus also having just been reading through EGG's Yggsburgh and seeing a completely different & IMO better approach, I may have reacted a bit too strongly to Matt's advice in particular. Because it is good advice for video game authors, and it's pretty good advice for writers of plot-based adventures for publication, too. It's just that IMO it's not such good advice for GMs at their own tables.
I haven't read Yggsburgh myself (everything I know about it is second hand) but it seems to me that might be setting the bar a bit high.
Different people have different DMing styles.
We have one DM in my group who's sort of a Yggsburgh type, though moreso regarding the big picture than the small. He has pages upon pages of detailed notes about his epic setting, which spans hundreds of thousands of years of time and two universes (as well as numerous alternate timelines). That's by no means to imply that things are set in stone. We've more than once overturned the "anticipated" result, thereby resulting in far flung changes to the setting. He's quite detail oriented.
Then there's arguably the best DM in our circle. He runs from maps. No real notes to speak of; he'll scribble the names, encounters, treasure, etc. right there on the map and just runs. He's by far the most flexible and improvisational DM I've ever gamed with, going so far as to allowing us to create a temporal paradox even though he'd never intended anything of that sort, and wasn't initially sure how to handle it. We paid a dear price for it, but he nonetheless ran with it where most DMs would likely have just said no, thereby creating one of the most memorable events in my group's history.
Then there's me. I won't lie; I started out as a crappy, railroading DM. In the beginning, I'd write out pages upon pages of adventure notes that I'd try to stick to like a script, going so far as to write out long monologues. In all fairness though, I constantly sought to improve my style, regularly reading about the art of DMing as well as studying DMs whose games I admired. Over time I improved significantly, both in offering open ended adventures and in streamlining my prep time. I've tried my hand at creating a Yggsburgh style game; it doesn't work for me. I lose interest in running the game long before I have the area fleshed out to that level of detail. I've learned that I need to prepare just a bit more than I'll need for my first game, then jump in feet first. That works for me. Nowadays I use a short bullet point list or a flowchart, jotting down important names, encounters, and drawing little maps. It's really just there to facilitate a lot of improvisation though. I don't force my players in any direction; they're free to do as they wish, even to the point of ignoring the adventure and doing something else (which rarely occurs, but when it does I improvise).
I think the matrix would actually help break DMs out of the scripted adventure mode of thinking, because it engenders a level of dynamism. One group might proceed through the adventure via encounters 1, 5, and 9, while another does 2, 3, 6, and 8. Yggsburgh might do the same, but I think it could discourage many DMs by seeming like an unattainable standard. Indeed, one could look at Yggburgh as being a massive, multi-dimensional matrix of encounters. Certainly more dynamic in it's design, but also less manageable for DMs without limitless free time on their hands.