“I don’t really understand the why being at zero growth doesn’t require the economy to be in a long term steady state”

That’s no problem. I can explain that very easily.

A zero growth economy can be extremely dynamic. Suppose we make 1 million cars a year, each valued at $20,000 and we provide $3000 of gasoline for each car. Then there’s a wonderful breakthrough in solar technology. The next year we make 1 million cars, each valued at $23,000 each — and these cars are entirely powered by the sun.

The economic growth between one year and the next could be zero, but the economy has still advanced very dramatically.