What is the default build configuration of cmake

target_link_libraries with optimized keyword corresponds to all configurations, which are not debug.

Try adding message(STATUS "Build type: ${CMAKE_BUILD_TYPE}") to your CMakeLists.txt to see the actual build type (I suppose it should be empty).

If depends on whether you are using a single-configuration generator (Makefiles) or a multi-configuration generator (Visual Studio, XCode).

The link cited in the question is about a multi-configuration generator. When using a multi-configuration generator, the configuration variable CMAKE_BUILD_TYPE is ignored. To select the configuration to build, cmake allows the switch --config, and this defaults to Debug. So

cmake --build .

in a multi-configuration project builds a Debug version.

However, when using a single-configuration generator, the switch --config is ignored. Only the configuration variable CMAKE_BUILD_TYPE is used to determine the build type, and this defaults to Release.

More background info on single- and multiconfiguration-generators in this answer.