IMHO, you should not have put words into their mouths about this being IDB.
In the true sense of the term as used by DOT, this is not IDB. They did not deny passage due to overselling the flight (either by physical seat count or by weight).
Side note, I'm assuming you're talking about a regional flight (...agent...was waiting outside on the ramp with other passengers to board the flight...). DOT rules are a little different (in the airline's favor) for small planes due to the fact that they can get "oversold" by weight much more frequently than a mainline jet. The seat count is a "hard" number, the allowable weight is highly variable.
Your complaint is that they had closed the flight ahead of the 15 minutes cutoff as specified in their Conditions of Carriage.
You have a valid complaint. And it is literally that they against your will, did not let you on their airplane, but the term IDB has specific meaning.
Customer Service Reps go thru how many complaints each day ?
They don't sincerely read thru each one ... they scan/research/respond/next item.
If you used the term IDB, I'd bet their scan focused on that one term and applied the industry/DOT specific definition. The research showed that this was not IDB, they offered a token "we're sorry", then moved on to the next complaint.