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It also says to make sure this condition is in a Group - but is that outdated?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?

You can see this LIVE here:

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!

You need to use the condition in the query builder.

So click this icon and select your $_GET.ev variable there. This means the filter will be applied only when the $_GET.ev returns a value, otherwise it will not be applied.