I would have politely reiterated "I'm a business select paying CUSTOMER. The ops agent directed me to board. I'm NOT a preboarder." If she pushed it after that, I would have moved, then written a letter to the corporate office regarding the specific flight attendant and her illogical actions, and demanded an explanation and refund of the extra payment for business select. Then I would put it (professionally) on twitter or whatever social media you use.

Sorry for the ridiculous treatment. If I would have witnessed that I would have done everything in my power (report to chief etc) to educate the FA that not everyone in a pilot uniform is a jumpseater that can be treated like a second class citizen. I know what it's like. I've had American employees do it to me a few times.