Correct. We have to restore the old SSD in DFU mode. Even if it was pulled from the machine, and reinserted.For example, let's say for example we buy a 2TB ssd kit, then when the installation goes wrong, the T2 event of the old SSD doesn't reboot, right?
Rerun the DFU restore if it refuses to boot again.
I have seen youtube video where the SSDs were swapped between two Mac Studios. But it failed because they were of different storage size. When the modules were put back in the original machine, they ran DFU restore again. And successfully installed macOS afterwards.
I sold my old 2TB kit to a macrumors user, that I pulled from my Mac Pro when I upgraded to 4TB.
He ran the DFU restore. When he turned the Mac Pro on, it showed an empty folder.
I believe he had to boot into Disk Utility and format the SSD. And then do an internet recovery.
He did not tell me what macOS version he installed. But he hasn't asked me for any help after that. So I assume he had no further problems with the installation.