I'm guessing the issue is that it's obviously "inspired" by "Terraform" which shorthand is TF.
My second guess is that using "Open" together with something that implies "Terraform" is also a problem, as Hashicorp might sue them for defamation, as that name would imply that Terraform is not "Open". I'm not personally against that, I wouldn't consider Terraform "Open" anymore, but I'm not sure a court would see it the same way.
The criteria for trademarks is not defamation, but confusion.
Given a product TerraForm, known also as TF, how likely is it that a consumer would be confused into thinking "OpenTF" is made by the same people who make TF?
That's the bar to clear. In the past, some trademark owners have been fairly lax, allowing unrelated "Open" versions of their software to be established. The classic case was "SUN OpenOffice", but I suspect they got away with it because "office" was not a wordmark - the trademark was for "Microsoft Office", which is very different. In a lot of cases, the "Open" versions emerged once the software was not sold anymore, so it was impossible to be confused.
In this case, though, the software is still sold and likely carries trademarks and wordmarks. Unless OpenTF's pockets are deep enough for some serious lawyerin', it will be easier to rebrand.
I'm not seeing any trademark registrations by Hashicorp on TF, nor any mentions of TF in any of the Terraform marketing pages. Not to say that Hashicorp couldn't try to be difficult anyway, but it feels like at this point doing this would just raise the visibility of the OpenTF project.
Tensorflow isn't a direct competitor. My understanding (and I am not a lawyer) is that enforcing trademarks is based around whether or not a consumer could reasonably confuse the product with the trademark owners. A user wouldn't reasonably confuse tensorflow with terraform, but might confuse openTF with terraform
