I think there is validity to this. I spent the greater part of my elementary school days walking to Scale Model Supplies, then the local Japan-o-shop, just for the sole purpose of buying Gundam models. I would then go home, assemble, and detail them. This was pretty much a daily activity for me. It got to the point where my two purpose built shelfs couldn't hold all the models, and I had to put some in storage, and throw some out. I built them all, including the 1/100 AND 5/100 (I think) scale Burning Gundams, both Deathscythe models, and about another 30+ from Gundam Seed (Justice and Guardian(this right?) being my favorite of that series). It surely didn't help with my financial situation, as I probably put well over $200 of my own money down for models.
But, I have gained a great deal of fine motor control. Now, I have moved on to bigger, more expensive hobbies, such as system building. I recently put my rig in a new case, something that takes most people about 2-3 hours, in about 30 minutes to an hour. I also can concentrate really well, and have much more patience for building things. If they indeed prove this correct, then I could certainly be a poster child for why Gundam models make you smart.