Well, they've always been very inconsistent on whether you can have overlapping parties. The usual PURPOSE of instance dungeons in server-based MMOs is that you can have as many instances as the server hardware allows, as long as players want to enter them. There have been a couple times they implied that someone would have to wait for Ye Xiu to get out of an instance in order to start, but there have also been new record clear time announcements that happened only a couple minutes before Ye Xiu set a new new record.
But I think this is the first time they announced the first kill of a sub-boss. Unless we're about to find out in the next chapter that it was actually a secret boss or the final boss of this dungeon.
im guessing the devs are implementing new feature like dungeon invader/raider like system such as from Where Winds Meet or Darksouls/Elden Ring that allow another party to enter their instance if the one initiating that instance dungeon cleared a few hidden checks/conditions that allow for such a thing to occurs.
then again, after i watched the 4th season of the anime (which skipped this part entirely) that time jumped to the final rounds of the challenger tournament where
Thrills faced off against Excellent Era and uh.... won
but i doubt the artist would deviate so much as to add these features in GLORY since this series is based off from the novel/web novel published years ago and i doubt the original author did implemented these kind of feature.
but i am open to be surprised...