It takes more than an unlocked bootloader to make Linux boot on random phones and work properly (and ensuring all the radios, camera, audio, phone calls etc work), and Volla have achieved that with their phones. I could be wrong, but I don't think it was possible to get a fully functional Linux distro going on any of these rebadged phones before Volla got to them.
replyVolla is just forwarding the trees made available by their upstream ODMs. E.g. Gigaset publishes them:
replyhttps://github.com/Gigaset-dev
I am not sure about the Daria Bond, but in Ubuntu Touch (which seems one of the very few Linux systems that supports the Daria Bond, ahem, Quintus), most of it seems to be the work of LineageOS developers (probably for generic Mediatek support, since it's a run-off-the-mill Mediatek phone), with some changes from Daria people on top of it.
So, I think you are giving credit to Volla that should go to the upstream ODMs and Lineage.
Or just go to the Volla about page:
https://volla.online/en/about/
It's just sales, marketing, and customer support people.