I didn't say this out loud when I was recording the video, but as I thought more, the question became:
"How to automatically remove ONE multi-select tag without removing all the other multi-selected tags?"
PS. My interim solution will be to separate the Status and the Tag relationship for that tag. I'll simply remove the automation update that extends the array with the 'Auto Exclude' tag. In other words, the status can change and the tag (I want removed in the video example) won't even be applied in the first place. On reflection this may be a more appropriate solution.
My question then loses it's priority business need and becomes more of a curiosity-based "I wonder how you do that" question.