HashiCorp makes its external contributors sign a CLA to basically hand over the copyright.
However the MPL and licensing in general is irrevocable. They have irrevocably licensed Terraform 1.5.5 under the MPL and an enterprise license (dual license). Anyone can use, modify and distribute version 1.5.5 under the terms of the MPL.
Since HashiCorp retains full copyright they can release the next version under the BSL.
Note that many free software projects (like Linux) don't have a CLA which makes relicensing impractical since every contributor would have to agree to it.