In the event that anyone comes across this, I seem to have found the answer.
I used this to clear out the service reminder:
Once I did, that particular metric was reset to 30,000 miles. So, it seems that it simply was a count-down reminder to check the brakes. Since it didn't go off by the time of the CPO process, they likely forgot to reset the value, thus it popped up just after I took delivery.