I (sort of) found the solution:
I found that I could read the reddit post by going to Google's cached version of the site:
And the Redditor u/Zren had a fix:
https://github.com/KDE/kwin/blob/master/kcmkwin/kwintabbox/kwintabboxsettings.kcfg#L37
Not sure where the GUI option is. Edit: Ah it's "Show selected window".
kwriteconfig5 --file="$HOME/.config/kwinrc" --group="TabBox" --key="HighlightWindows" "false"
qdbus org.kde.KWin /KWin reconfigure
Added the command my DelayTime=0 tip: https://zren.github.io/kde/#show-alt-tab-popup-as-fast-as-possible
But this still doesn't turn off the "popup" animation, just the background fade in animation.