EFI lock and SSD upgrade

I've been asked to upgrade this iMac 21.5" 2013 (model A1418) with an SSD because it's slow and I have to say it's not been going great.

I've cloned the slow HDD to a SSD using Carbon Copy Cloner 5 and also attempted CCC 6 both of which I have extended experience with. I've mounted the SSD in the iMac and upon boot a folder with question mark appears even though cloning was successful. I've successfully booted to the SSD when connecting it to other devices (with all of client's data there).

Now out of desperation I tried going into recovery (CMD+R) and it appears the imac has a firmware lock. It's very easy to say this is the reason it's not booting but then why does question mark icon appear (OS not found) and not just the lock?

Client has no idea what the EFI password is and he never set the EFI lock. He bought this brand new from big retailer and it was always in his posession.

Could the EFI lock prevent booting because it detects a hardware change (SSD)?