I think people need to take some of the specifics of who said what and look at what was actually said.
Two people got together for a meal. One complimented the other on the meal and asked how she was able to eat such rich food - not known for its slimming effect - and stay so slim - which in itself is another compliment to the host.
The host then responded by saying that she just ate small portions and worked out. If this were a genuine response, then she couldn't be faulted for it. If it were genuine then she could be seen as simply saying "I'm lucky, I have a high metabolism and am able to get away with eating this kind of food regularly so long as I work out."
If it were true. This isn't true, though. The host presented herself as something that wasn't true - she chose to lie. She was able to keep her figure - without getting into the eating disorder aspect of it all - because she wasn't eating the rich food. This becomes worse if you consider the fact that the guest was eating the rich food and had her own issues with weight that the host was very likely well aware of - the guests issues are her own, of course, and the host isn't under any obligation to provide a low fat option for the guest. But, the comment goes from being one of "well, I'm lucky..." to one of "I have more self control than you do," all while hiding the fact that she doesn't.
This makes Diana come across as a catty person. Regardless of what Diana's motivation was behind her words, she comes off as catty. Furthermore, given Oprah's well publicized weight problems, she also comes off as someone who isn't, in this moment at least, compassionate of other peoples' struggles - kind of ironically, given as this is an issue that Diana herself struggled with. If Diana was being compassionate in this moment, she wouldn't have lied. If Diana was being compassionate, she would have considered the fact that she was inviting someone who has publicly acknowledged her struggles with weight loss to lunch and served a low fat desert to her guest. But she wasn't being compassionate, she wasn't even being a good hostess. She was being catty, and this is being presented as a funny joke that she played. It's not, it's cruel and even kind of juvenile.