As long as any restaurant can have patrons wait to be seated and/or charge a no-show fee, that restaurant will honor late arrivals - particularly at Disney.
Why?
Because not honoring a reservation for folks arriving a few minutes late is utterly absurd, particularly at Customer Friendly Disney.
And because of the very distinction Disney makes about the difference between an ADR and a Reservation.
Arrive late and you will be seated - because when you arrive you are placed into the matrix that manages the dining queue. To be more specific, it does not really matter at all to Disney if you arrive late, because they have done nothing in anticipation of your arrival that would prevent them from honoring your late arrival; they are not 'holding' a table. Only way this does not work is of you missed the meal service entirely (i.e., arriving at 11:30 for breakfast).
And I'll give you but one example. Disney did not have my room ready at 4pm - where we planned to change into proper attire (that Disney allegedly requires). I had a 5pm ADR at CA Grill. Guess which Disney preferred - dirty clothes or providing Customer Service? I can absolutely promise you that Front Desk was able to get my ADR honored for my late arrival that occurred through no fault of my own.
Similarly, informing a Cast Member that the 20-minute Wait Time at Peter Pan turned into 30-minutes and made you late is going to have the same effect - an ADR that get's honored by a company that prides itself on Customer Service.
Apples and oranges there fella...
Sounds like you got some pixie dust, if anything.
It's interesting that you bring up a restaurant that you have to meet a dress code for vs a ride. But since you went there, in January, when we arrived in WDW, we came from the north where it was snowing. It was 80+ when we got to Orlando. The CM's at bell services had our luggage as our room wasn't ready. We asked if we could get a change of clothes, they allowed us to do so.
So, in theory, Disney actually let you slide, when you should've been held for the dress code, since you had the ability to change.
Again however, back to the topic..
It's not ok to show up late. Does it happen? Yes. Is it acceptable? Debatable... Depends on the situation... To me, if you're in the park, you should be there on time. If it's something extreme like you're actually physically stuck mid-ride, that's acceptable. If you are in the line but don't want to give up your spot... no, no sale. BOG doesn't have to worry about people missing because there's a line waiting to get in if someone doesn't show. Whether 1 minute or 1 hour, late is late. Sure, if you're off by a couple minutes, it shouldn't matter, but there is no language of any grace period allowable from Disney regarding ADR's. You can book them up to 180+10 days in advance... why in the world would you put one of the hardest to get ressies in that jeopardy.