This licensing regime is copied almost directly from GCC. The compiler part is GPL and the part that gets embedded into your program isn't. GPL-licensed compile

This licensing regime is copied almost directly from GCC. The compiler part is GPL and the part that gets embedded into your program isn't. GPL-licensed compilers need to do some kind of dance to allow the outputs from the compiler to be free of GPL restrictions, because typically some components distributed with the compiler are linked into the executable. In this case it's the standard library, so the standard library is MPL instead of GPL. Nothing to be concerned about here. I don't see anything unusual on this page.