I doubt they meant "no matter how hard you try" nor were limiting this to C programs only. Something the size of Firefox, for example, with hundreds of files, multiple languages and dozens of platform variations, is simply not designed for a fully static build and trying to modify it to build like that would be an absurd amount of work for basically no reason, so it hasn't been done and probably never will be.

Add licensing complications to that (Qt is notorious for this) and OP's statement is for all intents and purposes true.