Wrong….
You can run these as many times AS YOU NEED.
You can Get last boss but miss others and zone out find a new group even on mythic and enter there lockout and get the loot off the ones you missed.
The lockout only effects the bosses you were inside the instance for.
So if you were afk for all zone out and its in your mail if you missed 2/3 then new group.
you however can not get summoned in you need to manually walk in.