This is not a problem. It just means you’d be flashing a stock L4T R19.2 install. The R21.3 is highly recommended, and among R19.x, the R19.3 was much better than R19.2. The point I think that is being emphasized is that the same cable used for flash with the recovery mode entered just like you were going to flash is also the state used during cloning.
One suggestion you might experiment with…what you really need in terms of your own software is simply mmcblk0p1 root partition. If you clone that, you can loopback mount it and read or write to it from the clone on your host. This means you can also use the “apply_binaries.sh” from flashing R21.3 to this loopback mounted partition and at least in part bring it up to date to R21.3, then flash this updated partition with your original updates plus the apply_binaries.sh update (there is an option in flash.sh to re-use this partition copy during flash). Since you can copy this partition like a file and do not have to modify all of your copies of this partition you have a way of experimenting with at least some partial update of the L4T and not risking your original work.