Earlier quoted context omitted.
Absence of tax is not a subsidy. A subsidy is when something is cheaper than it should be because the government picks up some of the costs.
They are both taxed, but at a different rate. The government picks up the cost in terms of lost income. Yes, it's a subsidy (a deliberate tax break). The historic reason is that diesel was used for trucks. Later, diesel cars were considered to be environmentally better since they produce less CO2 per kilometer. They're just worse in other regards.
"a sum of money granted by the state or a public body to help an industry or business keep the price of a commodity or service low."
If the state is issuing a tax on something it is by definition not subsidized.