As I stated in that 2019 thread =
The U3419W driver page does not offer an Apple operating system driver. So we have no idea how the Apple keyboard can "talk" to the U3419W internal volume controller? You should start a new thread on the Apple Forum discussing this. Perhaps other Apple users can find an add in 3rd party workaround.
I'm not using an apple keyboard if that makes a difference, just regular media keys.
So I have to ask Apple to develop a driver for my Dell device? How big are the chances apple's developer team even get this request forwarded? So I spend 800 bucks for a device that doesn't have an apple driver?