I think the advice is relevant to the situation. What was she going to say? "get over yourself, they're happy and part of your family, just go and take them as they are"? While that would have been a good answer tolerance-wise, it might be lost on these people. If their problem is "how can we make sure our children don't think we think it's okay?" it's the appropriate answer.
I don't think Abby can or should educate these people on polyamory, it's not their role. While I find the answer disappointing, I think it's a good one, as advising that person to go would probably ruin the day for the rest of the family (and I guess for them as well).