I can't be bothered to download and run reflector on it but if they made it completely in C# then I don't see how they could have taken advantage of TCO since the compiler doesn't ever make use that CLR instruction.
I think you are confusing the clojure compiler and the clojure language compiler.
As far as I can see Clojure CLR uses the Dynamic Language Runtime and Expression Trees which do support tail recursion. I don't know if Clojure CLR takes advantage of it, probably not. However, recent .NET VMs automatically apply tail-recursion optimization if a number of specific conditions are met, even if the compiler doesn't actually emit tail-recursive IL.
Presumably the Clojure compiler emits MSIL. You can emit whatever instructions you want, even if the host language's compiler doesn't emit those instructions.