The message right now is technically correct: SourceTree for Windows only supports Git (and it does have limited Subversion support). The Windows client is still in beta, with Mercurial support coming in the near future. When it does gain Hg support, we'll update this copy.
If you're browsing on a Mac, you'll get a different message saying it supports Git, Mercurial, and Subversion.
If you have any other questions, let me know. Thanks!