Is everything I do in the same round "simultaneous?" If so, how do I open a door and walk through it in the same round? How do I draw my sword and attack with it?
You are applying real world definition to a game situation.
"Simultaneous" as used in the thread means occuring on your turn in the initiative order - since all of your actions occur on your turn (except for immediate actions) they are considered simultaneous in teh vague D&D combat system.
Otherwise when people move at what point in time do they pass through a threatend square? And how can someone move 30 feet in the same amount of time it takes to make a single attack with a sword. D&D combat system is very vague and that needs to be kept in mind when trying to do an analysis of the actions or else you will go crazy with it.