Type checking is a part of semantic analysis, which the author explicitly says isn’t present. Even so, the language it is compiling has only one type, so it is

Type checking is a part of semantic analysis, which the author explicitly says isn’t present. Even so, the language it is compiling has only one type, so it isn’t possible for a program to have type errors. Just like in C++ one never need to check if a reference is null—it simply can’t be expressed in the language, thus preventing any errors of that kind, a single type prevents all kinds of type errors. One might argue that this is the most powerful way to do type checking. It simply isn’t possible to have an error.