The assumption for the full renewable scenario are the existence of the hydrogen economy. I'm quite positive about the potential of hydrogen but there are quite a few unsolved problems at the moment and it seems the hydrogen part of the energy transition has slowed down a bit.
Certainly the total cost of such a system is not well known at this time. So the cost calculation for the renewable scenario is quite uncertain - other energy storage tech might be more expensive.
The carbon capture calculation is based on the assumption that there is no other renewables and we go 100% capture.
This is not really that interesting an scenario, what matters more is the marginal costs of each technology at different points in the future.
In general a healthy dose of scepticism is warented when it comes to long term projections or cost of technologies, though or course policy does require we take a stab at it.