You must not know any deaf people (although in D.C. you are near Gallaudet).
Some deaf people use both ASL and lip reading for comprehension of communication. In addition facial expression is useful to relay emotion. For that reason many interpreters sign and lip sync.
Her audience also was comprised, I supposed, of hearing people. Her lip synching the words helps them match the signs with the words.
If she was vocalizing like a hearing-impaired person (think Marlee Matlin in "Children of a Lesser God") you would have a point.
Yes, I know a 'few'. My spouse is Deaf, my first class in signing was at Gally, and one of my pet issues is disability rights. Since most of our interactions are with mixed hearing/Deaf crowds people are usually already talking at the same time as they are signing. If they are just signing though they do not try mouthing what they are saying. I rarely use lip movements when signing myself.
Yes, facial expressions do convey emotion. Expressions are not lip movements though.
In the video there is singing in a 'Deaf accent' going on. I am going to guess that was the soundtrack she was playing, but IN THE CONTEXT of her mouthing the words it changes any communication she is adding with lip movements to that of trying to lip sync to it.
i know couple of deaf people and they prefer lip reading to SL as far as communicating with "normal" people goes.
Most that I know say the exact opposite!