1. I agree, Facebook probably could have been more tactful in their reply. Your example reply looks good.

2. That said...if you were the security researcher, and you received a "This is not a bug." - you would still be fully wrong in selling the exploit to the highest bidder. It's not ethical to do that just because you failed to get a bounty after reporting it, especially if you only tried once.

I think both sides should have done things differently. Hacker News is skewed towards BigCo hatred as a whole, and I think it's showing a bit. The majority is siding with Khalil despite the fact that there are valid reasons for him to not receive a bug bounty.