I have lived here in the DR for 4 years now and have never, ever seen anything like this for something so insignificant.
Carjackings do happen during daylight:
http://www.dr1.com/forums/372844-post75.html
Car jackings have been happening in SD and Santiago a lot (it even happens in front of Colinas Mall in Santiago), but car jackers don't use passolas - which the OP described he saw.
Besides, with a car jacking and if the thieves didn't care about getting caught or anyone seeing it, and it clearly states that there were 7 of them and clearly outnumber the people in the car (and if they each had weapons as car jackers obviously use/carry them, it could've turned bad for the OP), that car would have been stolen - but it wasn't !
That's a busy highway, and it's clear that making an attempted car jacking with so many people (7) and using different methods of transportation ( bikes and passolas) is not the norm.