maybe i dont understand how articulated bus makes boarding faster. If anything it makes it slower compared to double decker .

Because they're longer, you can fit more doors on the side. If you allow all-door boarding, it's faster.

Double deckers I’ve seen have just one narrow staircase. You can’t have people go up and down at the same time. Through a door you can move some people in and out at the same time, however it’s rude, but since it’s a door and not a lengthy corridor, you can easily have a group of people go out, let new passengers in, and get in again.