There is a thing called acronym, abbreviation, or initialism copyright.
For Hashicorp's Terraform, "TF" is a well recognized abbreviation.
Hashicorp can't prohibit anyone from using "TF" in all areas, but it can argue that "OpenTF" confuses users that expect anything "TF" to mean "Terraform".
You can probably create a "OpenTF juice maker" without problem but OpenTofu especifically is in the same area as Hashicorp's Terraform, so it's better to avoid any trouble there.
There is a thing called acronym, abbreviation, or initialism copyright.
For Hashicorp's Terraform, "TF" is a well recognized abbreviation.
Hashicorp can't prohibit anyone from using "TF" in all areas, but it can argue that "OpenTF" confuses users that expect anything "TF" to mean "Terraform".
You can probably create a "OpenTF juice maker" without problem but OpenTofu especifically is in the same area as Hashicorp's Terraform, so it's better to avoid any trouble there.