I appreciate that you can't go far into technical details in an HN comment, but after a statement like that, you could at least give a pointer to those "good reasons". They might not be as obvious to everyone as you assume.
The power of Mustache (and Handlebars) was that it forced you to write logic-less templates. There's just no way to express logic other than checks for existence of values. That means that you have a minimal language that every non-programmer [1] can understand. You can change templates without diving into a language. Walrus changes that to be a pretty much full featured template language with full view logic (hey, look, there's MATH methods in there). It perverts the idea of Mustache.
[1] aka designer or template-guru :)