You're misreading what texti is all about. A comment is unix-style # and a heading (title) is using the wikipedia markup convention. You can put as many spaces before as you like it makes no difference to a comment or to a heading (title). See some samples @ https://github.com/texti/texti.github.io/tree/master/samples to compare markdown, texti and wikipedia markup using a real-world article (from wikipedia itself on markup languages).

About extensible - it's no different from markdown with two additions - 1) evolution / changes are more than welcome and 2) texti (like wikipedia markup) has (recursive) template extensions / includes built-in.