Yes, taxation is a core part of being a libertarian. Generally, libertarians believe in taxation only for essential government services, but policing is definitely considered essential to libertarians. It is central to the ideology.

Colloquially, anarcho-capitalism is sometimes also referred to as libertarianism. Those who subscribe to anarcho-capitalism believe in stateless societies, of which taxation would be incompatible. However, Thiel purports to be a libertarian in the stricter sense. This usage is not applicable here.