I think the author is talking about minor variations on Lisp syntax that include indentation-sensitivity, not about different languages that are homoiconic. Such variant syntaxes include i-expressions (http://srfi.schemers.org/srfi-49/srfi-49.html) and sweet-expressions (http://readable.sourceforge.net/), which the author mentions in the Acknowledgements. For example, here are some sweet-expressions:
define factorial(n)
if {n <= 1}
1
* n factorial(- n 1)
This desugars to:
(define (factorial n)
(if (<= n 1)
1
(* n (factorial (- n 1)))))
I think the Lisp power that the author is referring to is homoiconicity, and the corresponding ease of writing macros. For example, i-expressions sacrifice homoiconicity in some contexts.
But the author seems to have overlooked that sweet-expressions, which were developed later, do not sacrifice homoiconicity. A human reader can still easily understand the AST from reading sweet-expression syntax. The real reasons that sweet-expressions have faltered relate more to the increased difficulty of collaborating with other developers, the lack of editor plugins that highlight sweet-expressions correctly, and incompatibility with Clojure syntax.
But the author seems to have overlooked that sweet-expressions, which were developed later, do not sacrifice homoiconicity. A human reader can still easily understand the AST from reading sweet-expression syntax. The real reasons that sweet-expressions have faltered relate more to the increased difficulty of collaborating with other developers, the lack of editor plugins that highlight sweet-expressions correctly, and incompatibility with Clojure syntax.