The article doesn't ever explicitly say so, but Chen expects the reader to understand from his words that TASC was written in Applesoft BASIC!
Further, TASC's coverage of BASIC keywords is almost entirely complete, going by chapter 7 of the manual. Any given Applesoft program has a very high chance of successfully compiling out of the box, and the very few differences with the interpreted language can be easily worked around (if the `DIM` command is any indication). As one used to aftermarket compilers that only support subsets of the language, TASC's comprehensiveness is very, very impressive.
Yes the article does mention that TASC was written Applesoft
> Even if you hadn’t made it that deep into the manual, you could have figured out that TASC was used to compile itself because TASC used its own runtime library.
Further, TASC's coverage of BASIC keywords is almost entirely complete, going by chapter 7 of the manual. Any given Applesoft program has a very high chance of successfully compiling out of the box, and the very few differences with the interpreted language can be easily worked around (if the `DIM` command is any indication). As one used to aftermarket compilers that only support subsets of the language, TASC's comprehensiveness is very, very impressive.