I think Oprah is asked about them as a direct consequence of "The Interview", much to her chagrin. Like many, she's been Markled. She can't come out and say she regrets that bloody interview, so she snarks at them instead under the guise of a superficially cute anecdote. There's nothing cute about it though. She makes fun of their surname, pretending she can't pronounce it. That's PA. She makes it clear Harry refused to wait when she asked and inconvenienced her turning up, making him look arrogant. She makes him look like a complete fool in opening the car door so the mother duck flew out. It is they, not her, who acted badly towards those ducks. I think she is well aware of the cruelty of the situation.
She's canny. She knows what she's doing. They need her more than she needs them, if only to have social currency in Montecito. If/when the divorce tell all comes, Oprah will have her own stories to tell, so whether she gets that gig or not is probably not that big a deal to her.
Edit to add. For all Oprah's faults, she's self made from nothing and worked hard. I imagine she has quite a lot of contempt for 2 people who were offered so much in life, the sort of riches she had to graft for, and who have done so little with it. She may once have believed in them, but after 5 years of nothing burgers, I think she sees them as the talent free, busted flush that they are, hopelessly and without conviction copying others' success stories in the hope of short circuiting the hard yards portion of the process.