If that is true, Hashicorp can't change the license on any of the files that contain such code from MPL to BSL, as far as I understand. IANAL, but from what I understand of the license, it doesn't prevent them from distributing a binary with the BSL, but they would need to continue distributing any files with such contributions under the MPL along with any modifications to those files. And keeping track of which files would have that limitation sounds like a nightmare.