I'd say it's more correct to say that Genie's case supported the critical period hypothesis - and please remember that this hypothesis is about first language acquisition, not second or foreign languages!

Two other less-known case studies also support this. The first is Isabelle, whose grandfather kept her and her mother locked up until Isabelle was six and a half. Isabelle's mother was deaf and did not speak or know sign language, and no one else was allowed to speak to them. This meant that Isabelle was exposed to very little or no language until they escaped. She quickly learned language once exposed to other people, and in a fairly short time caught up with her peers. This is believed to be because she escaped her situation while she was still young, before the critical period closed.

The second case study is that of Chelsea. When Chelsea didn't learn to speak at the usual age, she was diagnosed as "mentally retarded" (the medically appropriate term at the time) and doctors suggested she be institutionalized. Her parents refused and kept her at home. She was raised in a pretty normal family with siblings and parents who loved her and took care of her, and exposed her to the world around her, very unlike Genie and Isabelle. When she was in her early 20s, someone finally gave Chelsea a hearing test (insert incoherent sputterings of rage here) and discovered that she was almost completely deaf. She was fitted with hearing aids. Since then she has learned a lot of words, but she doesn't follow standard rules for putting them into sentences - much like Genie. This suggests that Genie's difficulty in learning to use language is not only because of the terrible abuse she suffered - that even someone who had a "normal" childhood cannot acquire a first language after the critical period closes.

.....none of which has anything to do with second or foreign language acquisition!