That is exactly what reimplementation means in this context, creating a new implementation of an existing interface. And that is exactly what GNU Linux did. The

That is exactly what reimplementation means in this context, creating a new implementation of an existing interface. And that is exactly what GNU Linux did. They copied a large number of system header files and created new libraries that implemented them. The copied a large number of command-line interfaces, and created executables that implement them. They extended them in the process, but the goal was to mostly preserve API compatibility.