They would say that, wouldn’t they? But ready to go where? They probably didn’t have emonCMS in mind when that was written.
I didn’t understand your logic until the second post appeared. Now I think I do.
There is nothing permanent inside your Raspberry Pi. Think of the SD card as the equivalent of the HDD (or SSD) in your Laptop. The SD card IS the OS. Take it out of your RPi and you have nothing. It won’t boot, there’s no OS in the Pi itself. You didn’t install the OS in the Pi, you set up the OS on the SD card and commissioned it.
According to the post releasing the latest emonCMS emonSD-08May21 Beta release, it is based on Raspberry Pi OS, which is not the same as NOOBS. If you go down the route of using the install scripts to put emonCMS onto the NOOBS OS, I won’t be able to help you because I’m not a RPi expert, and I think generally the support available from us will not be as extensive as it would be if you stay with the May 21 image you’ve downloaded.
If you don’t want to overwrite NOOBS, you can always image it to your laptop and then re-use the SD card, or you can of course buy a new empty SD card and write the emonCMS image to that, or buy the pre-loaded SD card from The Shop.