While I understand that the game certainly hints at it very strongly, and it does, the fact remains that there's no actual text to officially say that. Without text, anything else is an educated guess. Those educated guesses might be correct, but without something actually saying it, it remains just that - conjecture. No matter how many things you can find to support your educated guess, without clear text it remains a guess.
For example.
Ghosts existing for each?
Who's to say the strategy wasn't officially "Players will choose a select few members, 3-4, to go in and stop the ghosts while others stay behind. Enough ghosts will be provided to let parties determine how many players go and try and provide enough time to reach a ghost depending on where they are located in the platform".
Heck, considering that depending on the order you come out in you can completely screw over the whole party, I could say that this design where healers pretty much always comes out last because they have to wait for the ghost to kill itself (while the train potentially destroys anyone else who came out before) would actually support the idea that they intended for at least 1 healer and 1 tank to stay behind for the party's survivability while the others are in the ghost phase and that the party-wide death is simply a dps check.
It's not unlike SE to make dps checks for only a subset of people affect the survivability of the entire party.
Without text to support it, this version of the strat would be equally as likely to have been intentional by the devs.
And like I said, the devs have admitted they watch world first clears, which this strat was used in many of them. If they were against it, they'd have said something by now. It wasn't a strategy introduced later and only by a small fraction of groups. It was widely used.
Now if they come out tomorrow with a post saying they do not want people to use this and to stop using it until they patch it, then I'll be right with you to call this an exploit.
And this is coming from a party that doesn't use this strategy at all because none of us in my party feel confident enough to rely on it.