Why is there a forced stealth section?

[Hezzlocks]

They absolutely do. When I did it, I sprinted by and made it to the door without any aggro before they had finished their banter.

10s later when the NPCs came through, both of the statues immediately aggroed to me. So after killing the statues, I made sure the NPCs were actually following me for the next pathing sequence.

I've seen someone do the same thing you did. But it wasn't the NPCs aggroing them, it was the one closest to the end door seeing him, since there's not really much of a "safe" spot by the door out of LoS of the one that paths there. It was quite obvious watching it, too. You aggro one, you aggro them all.