You're right
Written by Angela L
(9/15/2010 3:20 p.m.)
in consequence of the missive, Losing patience with Mary, penned by Isabelle M
I think JA wanted the reader to be angry at Mary but also fear for Edmund's happiness by being bewitched by her. As a reader you just have to groan.
But I also think it shows again the great art of JAs writing, to be able to describe this entire scene with all of the feelings that go with it, with these few lines.