It's a huge exploit that ruins PvP for a lot. It's harming those trying hard to get NF, recluse and so on. If you can't fix it (been in the game for a while) then why not just remove countdown early before shadow keep. Most of the community don't enjoy countdown anyway, hence it being removed in shadow keep ....
Iv had it in countdown where the round ends, only I spawn in the next round and we win. Then carries on as normal. I usually spawn in to all activities before my fire team (thank you SSD).
The only alternative to cheating in your vid is the same as above. Maybe 1 player loads in to the match first, no one else spawns and the round is over.
That’s clutching at straws, but just maybe a reason.
Or - they are glitching it somehow.
Either way, I respect your decision to bag at the end. Justified.