TBM, to me "Tax Exemption" means just that. A historic vehicle is not exempt from having road tax in order for it to be used on the road. It's just free of charge ATM, which is a whole different thing.
I agree with your logic John given you still have to "tax" historic vehicles, albeit at a zero rate. I guess this identifies those that are potentially used on the road as distinct from SORN as well allowing re-confirmation annually the vehicle complies with MOT exemption. Unfortunately though the government do use the term "vehicle tax exemption" in their online document referencing historic vehicles.