Counterpoint: if you think about it, yes it is.
To spell it out, based on the date, it's very weak evidence for something that happened many months before.
To spell it out in the way that doesn't require a crazy level of suspension of disbelief:
Based on the date (right after the public release of the assistant), it is actually very strong evidence for "we thought it would be awesome to have an AI voice companion that sounds like the one in Her", which, combined with the (undisputed) revelation that they indeed tried (twice) to get the person who did that voice, is a very strong indicator of the intent of the thing that happened many months before.
Yes, actually trying to hire Scarlett Johansson is very good evidence that they thought it would be awesome if they succeeded in hiring her as a celebrity voice. A one-word tweet adds nothing to that.
Tweeting “her” months later doesn’t prove anything. That Tweet might superficially look like evidence of intent, but if you think about it, it’s not.