In order to try the suggestions that Netgear has offered, you would need to change the wireless security on the AirPort Extreme from the recommended setting of WPA2 Personal to WPA/WPA2 Personal.....which is the same as WPA-PSK (TKIP).
This will lower the wireless security level on your AirPort Extreme.
If you decide to "test" out the recommendation, and the lowered security setting allows the Negear to connect, then you will have to decide whether you are willing to make that compromise.
If the Netgear extender still will not connect at the lowered security level settings, then I think you have to conclude that the changes that Netgear has made on the EX6200 have rendered it incompatible with the AirPort Extreme.
If you want to maintain the higher recommended security level on the AirPort Extreme of WPA2 Personal, then you might want to think about using another AirPort Extreme or possibly an AirPort Express, which will be totally compatible with the settings on the AirPort Extreme.
The AirPort Express...at least for now....is not "ac" compatible, so if you have an "ac" version of the AirPort Extreme and you want to be able to extend the faster "ac" signal, that would require another "ac" AirPort Extreme.