The article you linked to says:
So that setting is 2 channel PCM only.
Also you are using SPDIF, which can handle stereo PCM, or compressed 5.1 using AC3 or DTS. It won't support multichannel PCM.
In kodi, set channels to 2.0. Enable AC3 passthrough. Enable AC3 transcode. You will get multichannel output from your 5.1 speakers (it will be using AC3 as the actual format that goes over SPDIF).
You're a genius 👍 It works! Thank you very much.