Ford gave explicit instructions to imitate a copyrighted performance. Because that specific recording as owned by a record studio.
If you can describe a woman's voice and mannerisms and the result sounds similar to a copyrighted performance, that is natural circumstance.
If you want an example of purposefully imitating something with a copyright, look at GNU. Anyone who looked at the UNIX code was realistically prevented from writing their own kernel with similar functions. But if a handful of folks describe what the kernel ended up doing and some guy in his own head comes up with some C code and assembly to do end up with the same high level functions, well thats just fine, even if you include the original name.
The details matter. There is absolutely enough vocal difference, it doesn't take an audiologist to hear the two voices do sound different but very close. It would not be hard for the producers to describe "a" voice and that description would overlap heavily with ScarJo, and wow the marketing team reached out to see if she would attempt to fill the existing requirements. When she said no, they found a suitable alternative. If the intent was to have ScarJo do the voice and she said no and they did it anyways, thats illegal.