Yes, as your link states, it is the product manufacturers responsibility to release the code. And if they don't have it, they can sue Mediatek.
But very likely have access to the source, otherwise they couldn't adapt the kernel & co. to their boards. Soc is just one part of the whole board, full of other components that need kernel configurations...
But anyway, this thread it about the kernel, we talked about the bootloader and why it cannot be unlocked, which is a separate issue.
Manufacturer needs access to the bootloader to put their Android key for the image, which contains their special apps, in place. So they have sources. To be able to flash a different bootloader, they need to be able to fuse the bootloader key into the SOC, so they have a unlocked soc. So they have everything to offer unlockable bootloaders, if they care for it.