Managed to get this compiled and running on my M1 MBA pretty quickly, although a bit tricky.
For reference to those who might attempt, I had to swap 'python' for 'python3' in the header of the waf binary, patch one file to replace 'malloc.h' with 'stdlib.h" to overcome the one compile error, compile the hlsdk-portable library referenced in the README (again with waf tweaked), and copy both the resulting dlls/hl_arm64.dylib and cl_dll/client_arm64.dylib into the valve folder.
For reference to those who might attempt, I had to swap 'python' for 'python3' in the header of the waf binary, patch one file to replace 'malloc.h' with 'stdlib.h" to overcome the one compile error, compile the hlsdk-portable library referenced in the README (again with waf tweaked), and copy both the resulting dlls/hl_arm64.dylib and cl_dll/client_arm64.dylib into the valve folder.