I doubt they are using Johansson's voice. I expect they need much more studio-quality training data than they would have for her.
The desire to create a "Her" might be real but explains why they chose a similar voice actress, made Sky the default, and continued to pursue Johansson to some day create the real thing.
Suspending the Sky voice looks guilty but it might be a temporary action while the legal team considers their response. There might be a non-zero risk of being found liable if there were directions in the voice casting process to seek a result comparable to Scarlet Johansson. You'd want to collect and assess correspondence to see if that's a possibility, which might take a while.
Scarlett Johansson denied OpenAI the right to use her voice. They used it anyway. ( boingboing.net )