Changing the license in the repo does not change the license of files you have already distributed under the MPL. That would require them to revoke all those licenses, which they can't do under the terms of that license that they've already agreed to. This entire thread and OpenTF, and maybe all of OSS itself would probably not exist if that were not the case.
In the official repo I can still check out an older commit with the MPL on it, which means they are still distributing code under that license, but it wouldn't matter if that were the case or not under the law.
edit: In fact, software as a business ( and many other professions ) would not exist if license terms could be changed or revoked after distribution. License changes for new software to be distributed can change, sure. But short of a breach of contract as outlined in the license itself you cannot change a license you have granted.
That's not what they mean. They went through every file in the repository and updated the license to BUSL from MPL.
Unless they have approval from each contributor for a given file (or the contributor signed a CTA or CLA), they cannot re-license that particular file.
It's still MPL and only once all contributors approve or the MPL code is removed can it be re-licensed to an incompatible license.
Of course if they had wanted to move to a compatible license, for a lot of FOSS licenses that is an option. Regardless of whether that is an option for the MPL, they are moving to an incompatible license and could not do this.
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So their only options for files that they can't convert to the BUSL from the MPL are:
1. Keep contributions to those files under the MPL but require contributors to sign a CTA or CLA guaranteeing those contributions can later be relicensed.
2. Move BUSL contributions out of the file and only keep the MPL contributions in that file until they've all been replaced.
In the official repo I can still check out an older commit with the MPL on it, which means they are still distributing code under that license, but it wouldn't matter if that were the case or not under the law.
edit: In fact, software as a business ( and many other professions ) would not exist if license terms could be changed or revoked after distribution. License changes for new software to be distributed can change, sure. But short of a breach of contract as outlined in the license itself you cannot change a license you have granted.