This author misunderstands the basic argument: degrowth is not an alternative to growth - it is an alternative to catastrophic collapse. The point is not "the same amount of resources can support more happy human lives", the point is "wouldn't it be nice to keep our civilization going over the long term, instead of heedlessly running it off a cliff?"

Kinda leaves out the concept of overshoot, like we might not have a choice in the matter.

https://en.wikipedia.org/wiki/Ecological_overshoot

Yeah well the standard degrowth argument against the efficiency camp is that more efficiency has always led to more resource consumption, not less (rebound effect). At least in capitalist markets. The current 'AI' boom, which is very relevant to resource usage, was only possible because integration and energy efficiency of graphics cards became efficient to a degree that was enough for neural networks to do anything meaningful with text.