The note is providing the solution to unlock the system (primary/shadow) in emergency situations. If you are stuck with the upgrade and if you want to implement some corrections into the system you need to unlock the system.
Technically it is possible to bring in transport by interrupting the upgrade and unlocking the system and transporting the requests.
The other option is to setup the "dual maintenance" option if there is a need to do transports during an upgrade to the production system.
For more information check the SUM guide.
Regards
RB