This feature was about the last 100 messages processed from the Delta Chat folder automatically immediately after creating an account, even before the step 2 of your post where you import the key. This feature skipped all the messages it could not decipher, so in your scenario it would do nothing. Furthermore, this feature will be disabled in the next release and is already disabled in the preview release as discussed in Setting ‘No, chats only’ for ‘Show Classic E-Mails’ showing classic emails
In the scenario you describe, when you manually move the messages from the Delta Chat folder to the Inbox, all the messages will be processed by Delta Chat. There is by the way a simpler way: you can rename the DeltaChat folder, e.g. into “DeltaChatOld”, and wait for Delta Chat to scan it, because it periodically scans all the folders, but it may take a while.