Well i’m not entirely sure. I am not a Linux OS specialist.
I see what you are talking about with the ELK Audio OS Raspberry Pi binary images at the GitHub site you referenced. But i also see C++ source code for the Sushi and Sensei parts of the OS. If you look at the diagram of the “Software Components Inside an Elk Product” here, you will see that the Sushi and Sensei parts are “core Elk components”. There is a large portion of Elk that comes from other open sources for Embedded Linux created using the Yocto Project. (Read further down that same page.) Since Elk is using the latest of this Embedded Linux code, it should not be at the Elk GitHub site. (You see why right? There should be only one place for this.)
This all said i have only spent a few minutes to dig this far, so i am not totally sure on all this. And i certainly don’t fully understand the Dual licensing model. Perhaps someone else cam clear this up.