Glad you got it worked out, O2.
The manager is right--the guy screwed up. If you are in possession of a valid SIDA, secure area, or AOA badge, he should have been more helpful at the very least. I've had a few of those instances (as a manager--not as the guy denying entrance) that I've had to clear up.
Prior information is correct, you cannot piggyback off of someone into a sterile area, but you can share necessary information (changed gate codes) with someone who has gone through all the background checks, hoops, blood tests, IQ tests... etc. It's been my experience that gate codes get changed by FBOs when they get rid of somebody, in which case you (with valid ID) are not the issue.
EDIT: I should also add that employees, be they from the Executive Director down to the Janitor, are required to challenge anyone trying to seek access to sterile areas without valid ID OR those in sterile areas who do not have valid ID displayed. In that case, the guy in question should have pointed you to someone who could help you as opposed to accusing you of getting your license off the 'Net
-Brandon