I don't understand that either.
Usually, like 99% of the time, the limiting factor is the push and the towbar/towbarless disconnection.
Usually, if I wait until the ground guy says "pushback completed, set parking brake" to start the second engine (number 1 here), the engine start procedures and the thumbs up from the ground guy are obtained around the same time.
And that's with waiting. So I never lost any time by having to start both engines.
We don't have neos though
That could change the game indeed.
And I believe the neos can taxi without the APU too. That also changes a lot of things.