comma before adjective phrase: put people in a room, alone except for

[Julianus]

Hello.

1.a. Many years ago, psychologists performed an experiment in which they put a number of people in a room, alone except for a ring toss set. It was one of those children’s toys with a short wooden post held upright on the floor and a bunch of round rings. (from Korean university entrance exam)

I think that 'alone' modifies 'room' and 'alone except for a ring toss set' is essential. If so, why is 'comma' used before 'alone'? Is 'alone except for a ring toss set' non-essential?

Thank you always~.

 

[Egmont]

"Alone" (or the full phrase, "alone except for a ring toss set") describes the people, not the room. The only thing in the room, besides the people, was the ring toss set.

 

[Julianus]

Thank you, Enmont~.

But I can't still understand why comma is used before 'alone'.