Is this true? [...] Are there any demographical studies of LW's composition in personspace?

The closest things we have to those are probably the mid-2009 and late 2011 surveys. People could fill in their age, gender, race, profession, a few other things, and...politics!

The politics question had some default categories people could choose: libertarian, liberal, socialist, conservative & Communist. In 2009, 45% ticked the libertarian box, and in 2011, 32% (among the people who gave easy-to-categorize answers). Although those obviously aren't majorities, libertarianism is relatively popular here.

I mean, isn't that universally recognized as a mind killer?, just like most other political philosophies?

Political philosophies are like philosophies in general, I think. However mind-killy they are, a person can't really avoid having one; if they believe they don't have one, they usually have one they just don't know about.