well according to Julie she's "not sure"... I'd say that it migth rather be a bit of a slip by Austen. IIRC she and Cassandra tended to wear old fashioned gowns and adopted a "middle aged" style of dress... so possibly Austen thought of having a pocket as a useful place to put things, and mentions it, not thinking that a light muslin dress might nto have them....
I woudl imageiene though that warmer day dresses might have pockets if the wearer wanted them...and Emma, being a sensible woman and not a slave to fashion, woudl have one, sicne it was handy and useful. but I would imagine that her evening dresses were reasonably fashionable and well made....