Not every country uses a decommissioning subsidy as you should just take the money from the plants during their lifetime and invest it. This is what is happening in Finland and the fund already has more money then the projected decommissioning costs but they still keep pumping more money into them just in case.

They are accounting the tax money going from the government bank account into the owners of power plants. Money that could go to pay for hospitals or schools.

They don't account for theoretical subsidies of money, as then there would be more money in subsidies than there would be money. The theoretical subsidies that currently goes into fossil fuel to pay for global warming is reaching the point of infinitive money in the case that global warming goes beyond the tipping point. Theoretical subsidies are fun thought experiments but they aren't very valuable, as is infinitive money.

The report do account for decommissioning subsidy if there is actually government money going into the hands of power plants owners.