MSpec is in no way dependent on NUnit or xUnit. Thus, you can use MSpec yourself, as it has its own runners and approval methods. The blog posts you saw most likely used the "old" assertion library, depending neither on NUnit nor on xUnit. This dependency was removed at the end of 2009.
What you can do, although this is not required and I see no benefit in this, is to use the NUnit ( Assert.IsTrueand others) assertion library along with MSpec.
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