None of these required modification to the runtime, they were purely compiler features. And, as pointed out, they were features only Microsoft had the ability to implement.
(A better comparison would be to look at LINQ-to-SQL specifically; that would not be possible in any sense without the expression tree libraries and compiler support introduced in C# 3.0, since there was no way to "quote" a C# expression and look into it. That's much closer to the mark here.)
However, I agree with you in spirit. A big enough quantitative difference becomes a qualitative one; nobody would actually want to use LINQ+C#2. And likewise, few people want to write small-scale DSLs in C#.
But I would argue that since DSLs are all about affordances, it's not sufficient to say that similar functionality would be "possible". If the new approach doesn't represent significant semantic compression (which your hypothetical LINQ+C#2 would not), no one will use it. By that measure, the syntactic sugar added to C# 3.0 was absolutely a necessary precondition for LINQ.