Parks are often booked way in advance, often as soon as reservations open, which can be a year early, which may be why the en-suite cabins were booked. My guess is the Thursday in September is as close as they could get to a weekend with reasonable weather. I go with the idea the couple want a personalized yet budget approach, and I think it sounds like fun for those who can reasonably attend. I also think there is some cluelessness because people who are flying can't reasonably haul full camping gear with them. I would bet there are HomeAway, airbnb, and similar options available. If I could get the time off and had the money, I would look at those, or possibly renting a RV (I don't know if it is boat access only.) I would call somebody and ask, starting with the HC, but including the park itself. The people who work there probably get this question all the time. If I wanted to go, and could do so reasonably. If not, I'd decline. And not worry about it.
Yes, every choice establishes a priority, because it will inconvenience someone and someone else won't be able to attend, but I don't think that is rude, unless it is directed at a specific person.