Dropdown select field - trouble filtering on ALL condition

I’ve read this post: Dynamic Dropdown Filtering - include all option
and it still doesn’t make sense.

I don’t see how the condition is ignored when the GET value is null. (its null when ALL is selected).
It also says to make sure this condition is in a Group - but is that outdated? If there’s only 1 condition, why put it in a group?

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You can see this LIVE here: http://137.184.106.209/vehicle_list?ev=SHOW

This appears to be working for the Pulldown values, except for ALL.
Can someone please tell me how to get ALL working?
Thank you so much!