There was a third system, the fallback they used to prevent loss of life, legacy systems and a system shutdown.

You can argue thats not a sufficient third redundancy, I don't have enough context to throw an opinion out there on the safety of this particular infrastructure project.

I am confident that the fallback process was considered and kept in place as the tertiary backup.

Excellent point though. It took me some thinking to put myself back in SRM mindset and what they'd have considered here.