I suppose "disembark" literally means get off a boat (disboating? deboating?) but at least there's some logic, as it is the opposite of "embark". The other examples don't correspond to a verb such as "to plane" or "to platform", which is why they sound wrong. We can't deplane if we never planed in the first place.
I thought, many years ago, that my brother (who lives in the US) had made up deplaning. But then I was once at Newark airport and the announcer said "we are just pausing boarding while the other flight deplanes"