That use of quote marks around 'gentleman' is shockingly rude coming from a retailer. Yes, I do realise it is a side swipe at the original post; nevertheless, in retail the customer is always right, even if they're not.
I agree with much of your post, and others who have said similar things, but IMO the 'gentleman' jibe is shockingly rude coming from anyone. It's maybe more surprising coming from a retailer, and could be construed as a little unprofessional, but I don't see how it's any worse than the customers initial dig. The initial dig that no-one seemed to pick up on until the retailer did it! They're both in the wrong in my eyes.
And as for the "customer is always right", that's an outdated notion in retail now, for the most part. I don't even know if Selfridges use it any more! It was an admirable maxim, but society and retailing have changed to such an extent that it no longer makes sense. I'm sure there will be many articles on the web that explain why it no longer works,far more eloquently than I could! And adding "even when they're not" is just taking it too far 
As someone mentioned earlier, the truth is probably somewhere between the two stories we're expected to believe. In this instance I can quite understand the shop arguing it's case, as there's probably nothing to be gained from making a concilatory gesture. If they did so, it would possibly reinforce their supposed wrong-doing, and would undermine their member of staff. Many businesses nowadays will support their staff member, as the loss of a good staff member is more costly than the loss of a few potential sales.
The store obviously aren't toeing any lines, and they clearly accept the repercussions. That, to me, is more refreshing than a shop undermining it's staff, and apologising simply because they think it's the expected thing to do.