To what extent is the "war on terrorism" about 20th century Western values such as liberal democracy, economic liberalism, secularization?
The War on Terror is about these things only to the extent that the terrorist attacks are about them. The War on Terror itself is an attempt to prevent attacks on the US. This is something that is not really connected to Western values as all countries (including ones such as China and Russia) are concerned with preventing terrorist attacks in their territory.
To some extent, the Al-Qaeda attacks on the West do seem to be attacks on Western values. Terrorist groups at least say that they are eager to impose their version of Islamic theocracy on the world. This is a goal that is clearly opposed to secularization and probably opposed to liberal democracy and economic liberalism as well.
So, I would argue that the terrorist attacks were to some extent attacks on Western values and that the War on Terror is a response to those attacks.