Not true in an absolute sense, just not the FRG22D update -- and any other official ones will break your root as well as replace your custom recovery image. So, while you technically can use them, there's very little incentive to do so unless you have to run stock.
You can't "back up" stock recovery. The fastest and easiest way to get it back -- at present -- is to install the 79MB FRG01B update patch. Then you install whatever update you crave and re-root using this method. Or you could just install a rooted version of the OTA update, which generally hits within 24 hours of an update rolling out.