Sorry, I misinterpreted you asking for clarification on something that had been well-hashed-over in the thread for your not having read the thread. My bad.
I understand where she is coming from. The main reason is the line you said “I’m probably 20 years her junior, and I obviously know it’s an age thing”. I think if you referred to all females as ma’am then it wouldn’t be offensive. if you had a boss in her early 20’s would you still have used that word? If not then don’t use it for someone older. In this case it is clear you are commenting on and pointing out she is not that young by the usage of ma’am. I think if you are using a word to address an older woman that you wouldn’t a younger woman (or younger looking) than it’s best to stay away from that word. Age and beauty go hand in hand in our culture for females, so the word ma’am can be very hurtful. Obviously she must know she is a lot older than you, but she might not have been thinking and feeling it at that moment.
Nice! Very considerate. I am sure you have a ton of girlfriends!