Despite the many months that have passed since the original post, I will add my experience.

The wife and I are BA gold, and end up using India as an extended transit to reach SL. We have gone through DEL, BOM, HYD, and BLR airports. Each time, the check in was operated by a third party company, and before 2023, this company's staff were always unaware of Oneworld benefits.

The first time, we were told that we couldn't take 20kg extra each (= one extra suitcase each) despite flying business class and being Oneworld Emerald.

Several calls were made after we stood our ground. Then we were told that we had to show our gold cards (even though the boarding cards correctly showed our status), but my wife didn't have hers. Thankfully, the airport had free wifi so she downloaded the BA app to show some kind of acceptable gold card. I subsequently found out that all calls made by the check in staff were to their own management, and not to the airline.

At another airport, we had the same refusal but a lady with a Sri Lankan Airlines badge was hovering in the area so we just asked her to help.

Now, we print the page from the Flysmiles website and also from the Oneworld website clearly stating the extra luggage benefit. Other than that, you should always insist that the call be made to someone at Sri Lankan Airlines. Check in staff, especially when they are outsourced, are poorly trained on these matters and more often than not, they honestly have no clue what you are talking about.