Agree, I also would assume it's not possible. I don't care how much barrier you have between the pulling operation and someone standing near by, if the cable had an installation issue and whips off, it's not going to be enough protection.
But, that still doesn't stop a staggered opening. It would just mean some some downtime for completing each line.
Clearly, I have no actual knowledge. Which just makes me like 90% of the rest of us.
They could wait until all stations are done, all poles installed, all three lines pulled, and open everything in one big bang at one time.
They could also get the CBR building, DHS station, and the CBR/DHS line done first. Have a sort of "soft open" of just that line while the other stations and towers are all completed. Shut down and fall back to buses for a few days to string the POB/CBR line, and then soft open it too. Finally repeat the shutdown when they string the final EPCOT/CBR line.
That kind of staggered opening would let them work on procedures. Maybe even keeping the buses running at the same time, hence the "soft open". To me, the longest parts in the time line are the hotel station, since it's dependent on the new hotel work. And, stringing each line, since this probably needs large areas closed off while it's in process of being pulled. I'm imaging that they close down roads the line crosses and do not have guests in any rooms the line passes near. Probably creates havoc just getting to the rooms by road if you have to navigate around the line path during the actual installation time.
That kind of staggered installation would give them practice on the logistics starting with the easier areas, based on looking at a map. They could still install in this order and not actually open anything either until it's all done.
It's all just a guess. But, it's not technically impossible to open in stages.