my guess (hope? because I want it to work out for her lol) is that lunch is less special than dinner is and this is a special dinner for the
@Talby family... and that if she can some to see Sunday(saturday) dinner as worthy and special that her plans do not have to change very much at all. Disney draws the line between lunch and dinner arbitrarily especially now with their move to standardize lunch and dinner menus and prices. If it were a retirement area of Florida it would be 2pm! Seriously, it matters at very few places... H&V because characters change, some of the resort restaurants have changing menus still, some places do not have "lunch" at all... but the vast majority of park restaurants, dinner is when you make it. If it is your last major meal of the day followed by a delicious dessert party (who wants to eat dinner BEFORE dessert anyway?) I do believe that qualifies as dinner. Dinner is a state of mind, not a time of day. but IMO 3pm is way more dinner than lunch, especially with dessert at 5:45!
I think I have pretty much spent my arguments on this lol.