The History of the LISP Programming Language (1980 & 2023) [pdf]

"fully developed" does not mean "true", more like "complete".

For him a metasystem was a language system (just like a Lisp System is a List Processor System. LISP was actually the abbreviation for List Processor, not for List Processing), like a computer algebra system (CAS) with its own language, built on top of an implementation language. The first SCHEME system on top of a MACLISP system would be a Metasystem. The language SCHEME was not an extended Maclisp. The first ML would also be a Metasystem on top of, again, Lisp. ML was a complete Metasystem (funny, ML -> Meta Language), it had its own syntax, its own semantics, etc. It was first used as a part of a theorem prover, LCF.

Both SCHEME and ML were "fully developed", since they had their own syntax, had their own user interaction (-> a Scheme REPL or a ML interface). A CAS like REDUCE also has its own implementation language with its own syntax (the language is called RLISP), its own user interaction, etc.

Often these Metasystems would not be used from within Lisp (as an embedded language), but in a way, where the user does no longer interact with Lisp directly. For example in Macsysma, one gets into its own interaction interface and only can "escape" to Lisp via special commands or functions.

In Expert Systems implemented on top of Lisp in the 80s (and later), we have seen the same: some are embedded into Lisp (like KEE, the Knowledge Engineering Environment from Intellicorp, which can be loaded into a Lisp system, and developed as Lisp code) and some provide complete new programming environments (like GENSYM, an Expert System for real-time process control), with also a new language.

Herbert Stoyan then describes what kind of Metasystems had been developed in LISP until 1980.

His book was written, while he was still in the GDR (the German Democratic Republic, behind the "Wall"), in the late 1970s. It was published in the GDR. In 1981 he managed to go to West Germany, the Federal Republic of Germany. Since 1986 he was professor at the University of Konstanz, Germany. Later he was professor for AI in Erlangen/Germany. John McCarthy was visiting him there.