All those companies have very decent experience with Terraform. Even if some didn't contribute to the core, they all built decent software on top of or around Terraform.
In addition, we at Terramate also plan to contribute as much as possible. We have worked exhaustively with Terraform and related libraries such as HCL and are very well aware of the limitations and shortcomings we need to resolve with OpenTF.
I'd love to emphasize that many of us tried to contribute to Terraform in the past, but HashiCorp became somewhat hesitant to review and accept PRs which massively slowed down innovation for Terraform.
While I see the reasoning behind HashiCorps decision to switch licenses I strongly believe that closing up the ecosystem further won't do any good to Terraform and other of HashiCorps products, hence our strong buy-in for OpenTF.