I think what Nathan is referring to is the SQL-like syntax for LINQ, which really couldn't be implemented in C#, as C# doesn't have any syntax extension features.

However, expression-based LINQ is implemented in C#. That's possible because of several language features added to C# 3.0, notably expression trees (code as data) and anonymous functions (lambda). Those features make C# expressive enough to do things like LINQ in C#, though in a somewhat clumsier way than Clojure does it.

For example, if you write cities.Where(s => s.StartsWith("L")), that "s => " is a lambda expression, but because the Where method takes an expression tree, the expression is turned into a data structure rather than executable code. This is similar (not identical!) to Clojure recognizing that you're calling a macro rather than a function and letting you see code as data.