Please remember that this is in Closed Test. As a Concept Test, this may or may not move forward depending on the results of the test.
The section you're asking about is here:
The new mechanic can choose any plane within the squadron (as opposed to the current mechanic of always picking the last plane).
The plane that is picked will be focused until shot down or the specific target chosen may change if the planes leave all AA for some time.
The target can be within the Attacking Group (ie: the first planes in the squadron which will shoot).
The section you're asking about is here:
Because any plane can be selected, the planes shot down in the attacking group are not "replenished".
IE: If a Hakuryu is attacking with a squadron of 12 Torpedo Planes and the first plane shot down is in the Attacking Squadron, then the first Attack will only have 1 Plane.
If the 1 Plane remaining is also shot down before the attack happens, the attack goes on cooldown as the next Attacking Squadron readies up as the new Attacking Squadron.
This DevBlog is about a concept in Closed Testing which is only a conceptual test. I do not have specifics like exact cooldown time amounts.