I agree with him that there are people looking for "D&D Off-Ramps." I believe there are players who stick with D&D simply because they aren't clear on what direction to go away from the game. They might have a sense that what they want is something different from what they've been playing, but are unsure about taking a particular exit because they aren't clear on what they might find there. In that sense, I do believe that something like 13th Age is attractive as an off-ramp because it looks similar enough to D&D to be inviting, but different enough to potentially promise something different.
That being said, I also believe there are plenty of "Off-Ramps" which are not those games. In my opinion, as a gamer, I'm living in a time when new games -and new games of what I feel are good quality- are more accessible than ever. I also believe there are some gamers who have played (and possibly even highly enjoyed) D&D, but are looking for an experience which is not D&D or in any way related to D&D; in contrast, there may even be gamers (and I fall into this category) who enjoy the fluff and story elements of D&D, but have come to really not care for the mechanics of the game. In both cases, there are many Off-Ramps. In the past year, I've tried Dragon Age* and Star Wars: Edge of The Empire; prior to that, I've become a huge fan of GURPS 4th Edition. I remember a time when I would have been hesitant to buy products for a different game -for a variety of reasons. I'm now at a point where it's more likely that I'll choose something else over a WoTC product.
I do believe he is right. I think there are people looking for off-ramps. I also believe the games he chose are comfortable choices for off-ramps because they are similar enough to not be a drastic departure from a group's norm. However, I also believe there are people who are like I was a few years ago; looking for something which is decidedly not D&D or D20. For those people, there are a lot of great games to choose from right now beyond the three mentioned in his article. There may even be some people who like the D&D stories, but not the D&D mechanics, and there are even some games which can provide that.
(On a side note, I don't believe his use of GURPS as one of his examples was necessarily accurate. I understand what he was trying to say with the example, but I do not believe the example was accurate in light of what he chose to use as part of the example. The idea that D&D 3rd is both more modular and has more consistent rules than GURPS is somewhat laughable, and I say this as someone who used to love 3rd.)
(*Fun game, but I did come to have some problems with it, so it didn't last as a regular game with the group. On the other hand, SWs has become one of the group's primary games.)