Gimmick or not, it works for me for a few reasons.
First, any gimmick that gets me to eat more salad is neither simple nor dumb imo. Like Jim Gaffigan aptly calling out salad with bacon as a game of Find the Bacon, eating a well-designed and pretty healthy salad with a nice, warm everything bagel taste and crunch with each bite keeps me wanting more salad. I don't know if you watched the video showing how it is made, but the everything bagel pizza shell is topped with mozz and red onions and cooked in the oven first, then topped with a thin layer of sour cream, and then the salad is spread on top. So, a little different--and better imo--than merely tossing some everything bagel croutons in a salad. I imagine those croutons could take out a crown after a day or so, too.
Second, making a single-serving salad is a royal pain in the ass imo, and usually pretty wasteful because you can't buy only enough stuff to make a single serving of salad. Anytime I find a good prepared salad, I am happy.
Third, like any pizza topping, the slice itself is a good delivery mechanism. The thin layer of sour cream helps most of the salad stay put on the slice while you are eating it, which is another bonus because eating salad on the run can be tricky (and very frequently leads to dressing splatter on my shirt, tie and/or suit which is not easy to remove).
Fourth, and finally, the fact that it triggers so many people might make me like it even more.