It’s a little more complicated than that in PlatformIO. The basis is documented at Custom Platform & Board — PlatformIO latest documentation but then there’s also more platform-specific (here: platform-ststm32) involved in what boards are available, what Arduino core(s) is/are available and how it is built with SCons as the underlying build system.
So, it needs JSON board definitions, package files for the new Arduino core and Python code for building in order to pull in a new Arduino core which may have its very own special settings.
I’ll have a look at integration since it’s a long outsanding issue in that core.