Disclaimer: Advice is provided to the best of my knowledge but no implicit or explicit warranties are provided. Since the advisor explicitly encourages testing any and all suggestions on a test non-production environment advisor should not held liable or responsible for any actions taken based on the given advice.
In gneral, it can be open as long as you don't need to recover the system tablespace. You need to offline the affected tablespace before you can restore / recover it though. That error indicates that the tablespace being restored is still online.
However, for a general restore / recover, it can't be open.