To my understanding, this is not true. The "Sky" voice was based on a real voice actor they had hired months before contacting Johansson, with the casting call not mentioning anything about sounding like Johansson. [0]
I think it's plausible that they noticed some similarity and that's what prompted them to later reach out to see if they could get Johansson herself, but it's not Johansson's voice and does not appear to be someone hired to sound like her.
To my understanding, this is not true. The "Sky" voice was based on a real voice actor they had hired months before contacting Johansson, with the casting call not mentioning anything about sounding like Johansson. [0]
I think it's plausible that they noticed some similarity and that's what prompted them to later reach out to see if they could get Johansson herself, but it's not Johansson's voice and does not appear to be someone hired to sound like her.
[0]: https://archive.is/BNFvh