FMT_MODULE flag creates two different versions of the same CMake Target fmt::fmt
CMake, since version 3.28 supports installation of C++ Modules.
It is indeed possible to install and use the module version of the library by setting FMT_MODULE=ON before installing.
For widespread support for C++ Modules, we likely need a new CMake Target specific for modules, which can be installed alongside the regular fmt::fmt target and the fmt::fmt-header-only targets.