It's not her voice. But it may have been intended to sound like her voice. (I believe this less than twenty-four hours ago, but I'm hesitant to grant Altman the benefit of doubt.)
If it were her voice, would you agree that seems distasteful?
> one that sounds similar, but is different, and thus belongs to the true voice actress
They marketed it as her voice when Altman tweeted Her.
Even that is not open and shut. He tweeted one word. He certainly wanted an association between the product and the movie, but it is a much more specific assertion that that one word demonstrates an intent to associate the product's voice actress with the voice actress who portrayed the comparable product's voice actress in the movie.