I'm not really talking about the named people. The final DK quest literally has you waging war against the Argent Dawn and there is a ticker counting down how much of their army is left. It's not explicitly stated how many of the Argent Dawn's order is at Light's Hope Chapel in that battle, but pretty much every non-named member is killed there.
As for the order, the quest text clearly states it's a new thing so whatever, I don't want to argue in circles or anything, believe what you wish. You also need to keep in mind Fordring would not have the slightest clue about other members (who were mostly gathered in Stormwind during vanilla) as he spent all of his late-life time in E/WPL.
Let's be real here, those numbers don't matter and never have mattered. Look at the numbers they supposedly use for the blood elf/high elf population, and that was pre tempest keep/netherstorm/sunwell/shadowmoon/black temple elf genocide, yet there's still a billion blood/high elves in wow.