Regarding the technology comparison, that is true from a power system point of view - but for a utility which is making investment decisions in new generation capacity, it isn't so different. We include carbon taxes because coal power operators suffer that tax, it isn't an assumption about future policies. And equally to someone else's question about whether subsidies are included in solar costs, the answer is no. Because the LCOE is calculated from current module, balance of system and soft costs, and that while subsidies have brought the costs down, they aren't a component of the LCOE. As for system costs which perhaps you are referring to, that is fair from the system viewpoint but not particularly for the marginal unit of new capacity. UKERC have done some good research in the UK on system costs of renewables integration which perhaps you are referring to [0].
But you're absolutely right that there is a lot of nuance, especially around regional power market differences.
[0] https://www.carbonbrief.org/in-depth-whole-system-costs-rene...