This is a really misleading statement, especially since it comes after him stating that "LaTeX is free in both senses".
LaTeX input files apparently lack a standard specification, which is admittedly bad, but then again, so do the input files of many programming languages that some people on HN are writing on a daily basis[0].
This is not the same thing as them being proprietary; anyone can write a new LaTeX parser and there is nothing stopping them legally or technically from doing so.
[0] I may be wrong, but I believe PHP and Ruby both fall under this category. Markdown is another example (everyone parses it in a slightly different way, and while it's generally consistent, there are definitely warts around the edge cases where it's clear that Markdown would benefit from having a standard).