Can the Lagom.nl "Inversion / Pixel Walk" screen test truly indicate a problem on a 13" MacBook Air?
Every Mac I've ever owned does this same thing. Don't demand a replacement. 

As an aside, monitor flicker is sometimes noticeable because the 60Hz refresh rate is the same frequency as house electrical current. This causes your lights to blink at that rate. This is usually imperceptible, but can be noticeable with a monitor if they get synced up. This is more common with CRTs, but not unheard of with an LCD.

Don't worry yourself over this. Did you notice problems with your display before the test?