The parser's behavior is not specified except in the implementation, which is messy C. I was once loosely involved in a project that tried to reimplement it, which turns out to be extremely difficult, and was laughed at by Brent Welch. Indeed, it failed on countless edge cases. lex and yacc (or whatever) at least isolate this part of the language, making it much easier to have more than one implementation.

the point is not so much ease of implementation (a parser for a reasonable grammar is usually an insignificant part of a language implementation) as is homoiconicity, and thus easy extensibility. in this regard tcl is similar to lisp and forth (despite all three having different foundations).