Hi stringEM,
The addition of a serial Tx buffer to the Arduino SAMD core was proposed and implemented by Sandeep Mistry (samdeepmistry) way back in October 2015, but the change wasn't built into the core until July 2017. Which might explain why Adafruit haven't incorporated this change into their SAMD core code just yet.
If you're transmitting a lot of serial data then it's certainly worth adding, as it prevents the code from blocking each time you need to transmit a new character and really improves performance.
I was in the same position back in 2016 and ended up up just cutting 'n' pasting Sandeep's changes into my own copy of the Arduino SAMD core code.
The Github pull request changes required to add the Tx buffer are detailed here: Add TX buffer for Serial by sandeepmistry · Pull Request #58 · arduino/ArduinoCore-samd · GitHub.
Other additional serial port functionality that was also added at the same time (July 2017), and that may or may not be implemented by Adafruit, (I haven't checked), are detailed here: Add TX buffer for Serial by sandeepmistry · Pull Request #58 · arduino/ArduinoCore-samd · GitHub.