RPi: kept back firmware packages

Can anybody explain how dietpi handles the RPi related firmware packages?

after updateing to v149 and running apt update && apt upgrade i get the following:

The following packages have been kept back:
  libraspberrypi-bin libraspberrypi0 raspberrypi-bootloader raspberrypi-kernel

Does DietPi use some custom firmware packages out of the official Raspbian repos?

DietPi uses a custom kernel (branched from official RPi 4.4.x), with support for 384KHz audio output and support for Allo DAC’s.

Aside from the additional Allo firmware, we use the same firmware in our kernel package, as per Raspbian.

apt packages are placed on hold to prevent them overwriting our custom kernel.