Very few Republicans look like the 4 dweebs in the picture. The odds of getting 4 together for a picture....... I'm just not buying it.
The T-shirts are very obviously professionally created and printed, and are in three colors, and the quartet look like college students who dressed up to look like what they imagine Rs of their age would look. My guess is that the T-shirts were created and bought by the Sanders campaign, and the quartet are Sanders campaign workers.
As for Jeb's reaction, it was a damned if you do, damned if you don't moment. If he snarled at the quartet and stalked away, they had some one with a camera (probably 2 or 3) and could get several pix of their hurt helplessness and him in a huff. Or he could be a good sport, give them their chuckle, and move on.
I hope like Hades Jeb (and other RINOs) fades into nothingness, but just speaking tactically, playing along with them was easily the least harmful response. A snarl-and-huff response could have been grist for the MSM for a week; this pic probably got a chuckle on one day's AM and PM news before fading away.