The "Typemock Isolator for Open Source" edition is only free to use for developers of open source projects.
For Commercial users of any open source project that uses Typemock , it is not free, and costs based on what is said at the Typemock website.
If you are developing an open source application (such as SubText), Typemock is free to use for developing that specific application.
If you are a local user of that application (SubText in this case), you will need to buy a Typemock Isolator license. (you don't need to if it is hosted.. only if you install it locally)
If you are developing NHibernate functionality, Typemock is free to use to develop that functionlality.
A company that uses NHibernate will need to purchase a license to have that functionality available.