Thanks for all who posted, especially Ivanerr who posted a link regarding elite benefits specifically applies to JFK.

It seems like the gate agent was just following the rules that Ruby doesn't get priority boarding at JFK. BA is inconsistent when it comes to providing information. The 'Compare Tier Benefit' section of the BA webpage shows Priority Boarding is available to all elite levels (both BA and OW) but has two asterisks next to it saying not available for Sapphire and Ruby at London City airport; no mention of any other airport. However, the JFK airport information page on BA says priority boarding is not available to Ruby. What doesn't make sense the most is, the little information board at the boarding area at JFK had ovals representing all three OW elite levels.

The gate agent's answer was ambiguous. If he had said "no priority boarding for Ruby here" when I pointed out my status printed on the boarding pass, I would have believed him despite the announcement and information board at the boarding area said otherwise.

A few people mentioned about the logic behind insisting on getting priority boarding. As I said in original post, I wasn't frustrated because I didn't get priority boarding but because I was denied it against their announcement, against the information board and without a sensible answer.

Some sensed an accusatory tone in the post. However, I didn't mean any acquisition. I was just frustrated for the above reason. As a few people posted here noted, the stories of not well dressed, people with certain passports and ethnicity are looked down upon especially when using FF benefits or flying in premium cabin is not new in airline industry. For that reason, it was just a doubt not an acquisition. Now that it is clear, no more doubt and I learned something new.

Thanks again for all of your input. They are all valuable.

P.S. I am in Kenya now. Took another BA flight from London to Nairobi and I didn't even try to use priority boarding.