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.
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.
Doing otherwise is violating the MPL.