The "system" knows when you purchase your tickets using the CO/UAL CC... So when you want to check a free bag and/or board early (Priority Access) your Boarding Pass has it printed... if you use a different CC then you don't get any free bag/pre-board. I don't think this process starts until 3 March... reason you probably got away with it.

-----------from the CC website--------------
Free Checked Bag:
Free bag is for first checked bag for up to two people per reservation. Fees for additional/oversized/overweight bags will apply. Purchase of ticket(s) with MileagePlus® Explorer Business Card required. Get details.
Priority Boarding:
MileagePlus Explorer Card members will be granted boarding access to the plane after the boarding of Special Needs passengers, Elites and those with Premier Access designations, but before General Boarding of the remaining passengers. Benefits apply to the primary cardmember on the credit card account and anyone traveling on the same reservation. Benefits are available as long as the credit card account is open and not in default.Upon cancellation of the credit card account, all benefits will be cancelled. Chase is not responsible for the provision of or failure to provide the stated benefits.