I won't pretend to be a master political theoretician, so corrections are welcome, but at a high level here is what is going on.
In most of political theory, the spectrum breaks down roughly into two huge types of political party (and yes this is a massive generalization):
* those on the right wing, which are typically fiscally liberal and socially conservative, that is, they believe that companies should behave as they like, but people need to be told how to behave even in their private lives, and
* those on the left wing, which are mainly socially liberal, that is, they believe that the government should tax and regulate more heavily, but that people should be able to do what they like privately.
Across the globe what it means to be "a liberal" is different. In the US, the "liberal" signifier refers to the fact that the person believes that the government should not regulate personal decisions like gay marriage. In France, it is the opposite: the "liberal" signifer refers to the fact that they like less regulation on the markets.
The Economist, simply put, is liberal in the sense that it tends to favor less regulation on both these fronts: it favors legalization of drugs and gay marriage, but also less regulation for markets.
There are of course exceptions (it favors gun regulation for example), but this is basically the position that it has come to call "radical centerism". The magazine is often called libertarian, but only conveniently so -- the gun control exception in particular casts this supposition into doubt, and rather than accept this branding, the editorial staff has evidently decided to pick this new title.
This is probably the right move. For all its flaws, this is something that the article did get right.