This was back in 2011. We modified the init script to stop the zygote process from launching, then used our own C code running as a normal Linux process to work with /dev/fb. So yeah it was rooted and it was an old version, but we were mostly outside of the Android ecosystem entirely.
Starting with Android N, Google has been locking down what is possible to do via the NDK and apps can no longer directly read outside their own sandboxed filesystem, so no /dev.