Well, mostly means exactly that.

My AI will always attack the closest enemy units first.
Except the targetmanager has a high priority target like nuke, paragon or incoming experimental.

There are also 2 different attack vectors for AIR experimentals.
The 1st also attacks the closest enemy, the second tries to fly in the middle of the enemy base.

Have also in mind there are different AIs with different tactics.
Did you test Relentors RNGAI or the Swarm AI from Azrael ?

You can join us on Discord (link at bottom of my AI post) for more information.