Yes, it's an operating cost, and every time you have a tax exemption, it's a subsidy according to the WTO, because it's explicitly written.
I would like to add another thing, please. When you discuss the subsidies in the WTO, you have to take the technical definitions, because sometimes economists for their own reasons take a very broad approach to what a subsidy is. But from a legal point of view, of course, you have to have the technical definition of subsidies, and that means you have a financial contribution by the government, and you have a benefit for the recipient. For example, a tax exemption is clearly a subsidy, so it would be in the red box without any doubt.