The author later states that std as a module isn't implemented in standard C++20 (though hopefully will be in C++23) and provides ways to work around that somewhere in the back of the book. It's somewhere in the appendix.

There are also experimental flags to enable (some parts of) this behaviour on the most recent compilers, apparently. (i.e. https://learn.microsoft.com/en-us/cpp/cpp/modules-cpp?view=m...)

Clang doesn't seem to support modules well and g++ is rapidly falling behind the other compilers. It'll be a few years before this code will compile on any system of your choosing. I'm confident Microsoft can release a compiler that will compile all the code mentioned once C++23 gets standardised, though.

Kind of weird to write a book using a language standard that's not even out yet, especially in a language where the freely available compilers lack a significant portion of the standard, but as one of the language's designers I can see why he's getting ahead of himself.

It's probably better to have this book available already once C++23 does finally land than to have to wait for standardisation before writing a book using it, but I think the book shouldn't have relied on unreleased standards or custom setups like it does until the new standard is out. Yes, you can get a std module of your own and yes the performance improvements are significant, but with open toolsets lagging behind I don't think it's smart to merely mention the downsides in a side note and the appendix.