The statement that C++ modules cannot be built in parallel is not entirely accurate. While C++ modules introduce a new system of organizing and managing code that can affect build processes, they do not inherently prevent parallel builds.

In traditional C++ compilation, each translation unit (source file) is compiled independently, which allows for parallel compilation. With modules, the relationship between modules can create dependencies that might require a more careful approach to parallelization. Specifically, if one module depends on another, the dependent module must be built after the module it relies on. This can lead to scenarios where dependencies need to be resolved before some modules can be compiled.

However, many modern build systems are designed to manage these dependencies effectively, allowing for parallel compilation when possible. They can analyze the module dependencies and optimize the build process accordingly.

In summary, while C++ modules introduce some complexities regarding dependencies, they do not eliminate the possibility of parallel builds. With a well-configured build system, you can still take advantage of parallel compilation to speed up the build process.