I noticed Mattairtech's pin definitions include PER_ATTR_DRIVE_STRONG on them. But in googling that, the only reference I find to that constant is on Mattairtech's own page.
I know the Zero's specs state 7mA is the max per pin, but I also know that the SAMD21 has two different drive strengths you can specify and only the strong drive strength allows for 7mA per pin. But what I don't know is where I would look in the Zero's source to verify that the pin drive strength is actually being set properly.
I also just searched for references to the register to set drive strength and I found this: https://github.com/arduino/ArduinoCore-samd/issues/158
So it would appear that as I feared, the specs for the Zero that state the pins can source/sink 7mA are incorrect or at least, misleading as things are currently configured.