Have to use SerialUSB on M0 to print to Serial Monitor, but variant.h says otherwise

Here is the relevant part of `variant.h`:
https://github.com/arduino/ArduinoCore-samd/blob/bab730bf6e4716601fe51cbb692d9b25cea90386/variants/arduino_mzero/variant.h#L203

Am I doing something wrong?

Start with variants/arduino_mzero/variant.h at the linked lines and compare the Serial and SerialUSB definitions. Reproduce the Serial Monitor behavior on an M0 board, then verify which interface the variant should expose and document the confirmed behavior or correction.

Written by the indexing model from the issue text.