How do come to that conclusion? The bulk of the book uses OCaml for examples and defines a simple language called F-flat:
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2.3 The F[ Programming Language
Now that we have seen how to define and understand operational semantics, we will begin to study our first programming language: F[.
F[ is a shrunk (flattened) pure functional programming language.
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Sorry about the missing flat symbol, it didn't survive copy and paste from the pdf.