Late lunch is perfectly acceptable. I'd even argue that a late dinner reservation, at Disney, is perfectly acceptable as well.
Now at home, I would never go into a table service restaurant less than an hour before closing. Everyone wants to go home and showing up asking them to re-clean their kitchen equipment is a no fun way to get a lackluster meal. Better off finding a grocery store or fast food for late night eats.
I think it's more acceptable at Disney due to the highly competitive nature of dining reservations and the fact that Disney wouldn't offer later dinner reservations if they didn't have the capacity for it. I'm not saying to lollygag after eating a late dinner, but I'm saying that since you're at Disney, and have a reservation, it's perfectly acceptable for you to take the normal amount of time it takes you and yours to eat a meal.