You and the comment to which you respond both seem to be mistaking the specific example on the sweet.js homepage, which is explicitly described as providing ES6-standard functionality early, for the limit of the possibilities inherent in a hygienic macro system. This is a pretty grave mistake, in that it makes understanding the proposition at hand nearly impossible.

sweet.js is more or less a Lisp macro system for Javascript. Gaining some familiarity with Lisp macros would probably serve you well in terms of being better able to understand what's on offer here and how useful it can potentially be.