Why? It's correct. You run a tool to convert the Web source to Pascal. If you want to compile TeX on modern computers, you run another conversion step to convert the Pascal to C, but Knuth used a real Pascal compiler when he made TeX.
https://books.google.com/books?id=_tgh4bgQ6PAC&printsec=fron...
Compilers are computer programs that translate a program written in one language into a program written in another language.