To people saying that writing a compiler is almost always an overkill - that's because you and author mean different things by the word "compiler". Or, in other words, that's because your intuition is based on the inflexible languages you're working with.
It's not an accident that the talk/article is about Clojure[0]. A Lisp. In Lisp, "writing a compiler" is a fancier way of saying "writing a possibly code-walking macro", which is still just a fancy way of saying "writing a function that transforms a tree into another tree, and having it run compile-time on parsed program source as the input". 99% of the usual compiler work - tokenizing, building an AST, generating machine/bytecode, optimizing - is handled for you by the host Lisp compiler. All the mechanics are there, and all you have to do is to add the brains you need.
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[0] - Clojure itself actually makes this slightly more difficult than it looks, because of the decisions it made that makes the syntax slightly less obviously homoiconic.
It's not an accident that the talk/article is about Clojure[0]. A Lisp. In Lisp, "writing a compiler" is a fancier way of saying "writing a possibly code-walking macro", which is still just a fancy way of saying "writing a function that transforms a tree into another tree, and having it run compile-time on parsed program source as the input". 99% of the usual compiler work - tokenizing, building an AST, generating machine/bytecode, optimizing - is handled for you by the host Lisp compiler. All the mechanics are there, and all you have to do is to add the brains you need.
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[0] - Clojure itself actually makes this slightly more difficult than it looks, because of the decisions it made that makes the syntax slightly less obviously homoiconic.