Contributors rights are being violated then. This would only be legal if ownership over contributions was transferred via a CLA (Contributor License Agreement).
It doesn’l look like they have one even now (look at audacity for example which do have one), so I assume they had no CLA prior to this and every contributors rights are being violated by including their code in a closed license project.
There could naturally also be deals made with contributors to sign over those rights, there have been projects in the past that got enough developers to sign their contributions over and rewrote the rest. Doubt this makes sense for a medium-scale project like this tho.
Needs a none option for base