I don't think he understands what the word "proprietary" means. It means that the file format is secret or there are legal constraints on its use. Until recently Word documents clearly counted as prorietary, but now that microsoft has been forced to document its format to some degree .doc is listed as "controversially" proprietary on wikipedia.
Even today you will need to buy Microsoft Word to "properly" read .doc files. There are other readers (libreoffice) but they only do the basics, and in my experience they usually mangle the file.
LaTeX has never been proprietary because it has always been publicly documented.
The point is, are there any other applications that will render a LaTeX document correctly that aren't LaTeX itself? It's open source, but in practice the code is so complicated and large no one has ever duplicated it. It's not proprietary in the traditional sense, but if you want your LaTeX file to not be mangled, you must use LaTeX itself.
There are many programs that implement subsets of LaTeX/TeX. For example, for math layout I believe Mathjax and matplotlib have both essentially copied Knuth's program, based on his "TeXBook" and "TeX: The Program" books, which document the TeX code extremely thoroughly using his "literate programming" technique. TeX is one of the best documented programs in existence.
Google "Latex implementation" and you will see a lot of hits. I see a Java implementation, Windows implementations, LaTeX3 and LuaTeX are referred to as reimplemetations 'nearly from scratch', etc.
And here's the problem: all of them are mutually incompatible in most situations. Even moving .tex documents over different platforms is an enormous pain, and pdflatex/xelatex don't possess the error reporting you need to quickly find which packages are missing on which systems.
Don't get me wrong, I love LaTeX, and I agree that the author misinterprets 'proprietary', but from a user standpoint the problem is the same: old documents are not rendered correctly, and new documents don't work with old compilers. It's a mess.
> all of them are mutually incompatible in most situations.
I haven't compiled against every implementation, but I did just recently rerun a report I create 4 years ago under a different engine. I spent about 20 minutes addressing the new complaints, when I finished that the generated pdf looked exactly the same as the old one - but with up to date data. Try that with html :) Heck, I've had the exact same experience switching C compilers. I do agree with you about the crappy error reporting. There is a reason why the Library of Congress is bundling data with binaries now, this is a very common problem - but in my experience Latex has fared much better than most formats.
http://en.wikipedia.org/wiki/Proprietary_format
Even today you will need to buy Microsoft Word to "properly" read .doc files. There are other readers (libreoffice) but they only do the basics, and in my experience they usually mangle the file.
LaTeX has never been proprietary because it has always been publicly documented.