While this debate has been mostly “I’m right” or “you’re wrong”, like most things this isn’t a black and white issue.
If one of the objectors to this post was at the theatre and someone “less fortunate than themselves” was making so much noise that much of the audience couldn’t hear a word of what was being said on stage, I think they’d be more likely to protest, no matter how much this less fortunate person was enjoying themselves.
Similarly, if the people in Mark’s post were only occasionally making a murmur, I doubt he’d have a problem with their presence.
I don’t think any of us can judge exactly how we’d feel without having been witness to the events Mark describes. You can disapprove, but unless you know how disruptive this was, it’s a bit much to offer an outraged dismissal and say anything goes.