That's not "not being offended". That's dealing with the issue. Two completely different things. You're displaying a horribly inaccurate and actively harmful attitude that used the be common of "nobody can make you feel anything you don't want to feel". That's a form of gaslighting
Nope. I'm going to correct factually incorrect information, especially when it's damaging misconceptions like the one he stated. "It's just my opinion" is not an accurate statement, either, nor is it a defense of repeating such falsehoods. Further, the "you choose to be offended" sentiment is used all too frequently by assholes to defend their asshole behavior. "It's not MY fault you were offended by that horribly dickish thing that I said to you! You chose to be offended!" I'm going to shut that nonsense down whenever I see it.
....the rudeness the OP got WAS the topic at hand.
Absolutely always true. Seems that, like Shard, you're conflating whether someone is offended with how they respond to being offended. Handling it without complaint isn't the same as not being offended. They're still offended, they're just not complaining about it.