If the engineers don't design the laptops, then who does? Please note that I did say that the designs have to be approved by Apple's management, but the engineers determine what the system looks like and what goes in them. Assuming their designs would be approved, they are ones who would know.
Now if management declines them, thats another issue entirely.
You have it backwards. No engineering team is going to get a substantial budget to build a machine and then later have management reject it.
What more likely happens is that management/marketing/concept designers come to them with a prototype device and the engineers are asked if they can pack the correct innards/functionality into the device. Ideally that group would be coming to the engineers with the "problems"/"needs" that users have that need a solution. However, if is also likely that is were the fanatic "thin is super important" comes from to though.
The process doesn't "start" with the engineers. That is where you are off the track. They are essential to a production product but they are not necessarily driving the selection of which market segments for which products that ship.
There may be R&D projects where they are given a budget to create a concept device (that is never going to ship) but that also is substantially different. Those are research projects that need some development to construct the prototype ( R&d) . Not development projects that need some research (r&D) . The vast majority of Apple is spent on r&D projects. There is extremely little revolutionary research being done at Apple now.
At one time I think there was a process at Apple where various teams went off and built competing overlapping concepts and management tried to pick the "best" set of winners from that to ship. That's when the R&D budget was much higher and Apple didn't quite do as well as they are doing now.