I’m not sure the thread @darwindesign linked is relevant to what you are saying. That was about the number of HDMI channels needed for various passthrough formats.
Regarding LPCM, there is no such thing as a ‘7.1 container’. HDMI has two modes: it carries 4 stereo channels which either contain 4 successive stereo samples or 4 simultaneous ‘stereo’ samples to provide up to 8 channels. I guess you could call that a 7.1 container at a stretch. For 5.1, six channels are carried in three of those ‘stereo’ pairs and one pair is not used. 3/4 years ago we did a lot of work to make sure the spare pair of channels was properly muted because users were getting funny noises on RL and RR when playing 5.1. Maybe that’s what you remember.
Passthrough formats similarly use 4 successive 2-channel samples (DD, DD+, DTS) or all 8 channels (HD audio formats). But that has nothing to do with the number of channels you set in Kodi’s settings unless you want to use the AC3 transcoding option.