ELI5 for people who don't know what project sundial is because I didn't know about it until I saw that video:
In the 1950's the guy who invented the Hydrogen nuclear bomb made a theoretical design for a bomb that would send the entire planet into a nuclear winter. The bomb would be detonated in somewhere in the US because it was so powerful that both the US and whoever on the globe they were trying to hit would get destroyed either way.
I know stories about a scientists that made a "weapon vs. delivery method" table, where the delivery method first would grow and grow, then it started to shrink until it became "backyard" because it made no difference where the weapon exploded. I have no idea if those were related, and how often people came to that conclusion that "hey, if we increase our bombs just this much, they don't need to be bombs anymore".
I'm going to assume no to your first part since a lot of the documents surrounding the project are only being declassified now (and a lot of the details are still classified apparently)
To your second part, I was paraphrasing but the video I linked also called it a "backyard bomb" so the the project was probably being worked on around the time the terminology for that "scale" was made