The tax is specifically levied on vehicles that are on the road. That much is inarguable.
The amount you are charged is then set at the amount you pollute, and very low polluters like electric cars (and of course any non-motor traffic such as bicycles) have the tax waived entirely.
So two different things. What it's for, and how much you pay.
What it specifically *isn't* is a tax that pays *for* roads. Unfortunately the term "road tax" confuses the issue in people's heads (such as the author of this article).