How does it seem to know which zergling is about to get hit?

Because the tanks are using the default unit AI, which AFAIK amounts to "when your weapon is ready to fire, shoot the closest enemy in range". The zergling AI knows the attack range and rate of each tank so can ensure that the closest zergling is always on its own when a tank is ready to fire.

What would be really interesting is putting a similar smart AI on the tanks, which could make decisions like shooting at the highest concentration of zerglings instead of the closest one, or holding its fire if one lone zergling is in range, but a clump of them is about to enter range.

Human players already do both optimizations with widow mines, which are a unit that takes several seconds to prepare its shot so it's feasible to retarget. It's not feasible for humans to do this with tanks though as they fire immediately and automatically.

You could even see counterevolution as two AI algorithms become better at predicting what the other will do.

Game theory dictates that you shouldn't anticipate what your opponent is likely to do, but rather assume that they will chose the best choice available. Given that you need to move the zerglings before the tank fires the tank should be able to retarget and always hit the highest concentration. That being said organizing your lings to run in a formation to never expose any to splash shouldn't be too hard.

If you slow it down, it would be apparent to you too. High-level human players do this kind of thing to, just not this fast. But effectively, it's watching where the attacking unit is facing and it knows when the next attack is coming, so it knows what the viable targets are and can react when it becomes obvious.

(it's also possible the system is receiving information directly from the game engine, but it's definitely possible to figure this out from nothing but the information on screen)

And that was created through the games editor I believe. Imagine how much better they can make something with full support for AI development.

Unbelievable! From 2011 even! That's awesome. Can't wait to see more detail on this going forward.