I think you are correct, but you probably misunderstood the parent.

My understanding of what they meant by "retroactively apply a restrictive license" is to apply a restrictive license to previous commits that were already distributed using a FOSS license (the FOSS part being implied by the new license being "restrictive" and because these discussions are usually around license changes for previously FOSS projects such as Terraform).

As allowing redistribution under at least the same license is usually a requirement for a license to be considered FOSS, you can't really change the license of an existing version as anyone who has acquired the version under the previous license can still redistribute it under the same terms.

Edit: s/commit/version/, added "under the same terms" at the end, add that the new license being "restrictive" contributes to the implication that the previous license was FOSS