The argument is that the wooly mammoth actually helps counteract the release of greenhouse gasses into the atmosphere:
"When the herds of northern megaherbivores were killed off by humans ten millennia ago, Zimov says, the largest biome on earth, called the “mammoth steppe,” converted from grassland to boreal forest and tundra. In these days of global warming, thawing tundra is releasing greenhouse gases, whereas grassland fixes carbon." [0]
There is a really great documentary about Stewart Brand, We Are as Gods, that also covers this project. [1]
I had the same question - it appears the We Are as Gods documentary is not out yet. According to IMDB, it will be released in March 2021 at the SXSW film festival (March 16-20).
"When the herds of northern megaherbivores were killed off by humans ten millennia ago, Zimov says, the largest biome on earth, called the “mammoth steppe,” converted from grassland to boreal forest and tundra. In these days of global warming, thawing tundra is releasing greenhouse gases, whereas grassland fixes carbon." [0]
There is a really great documentary about Stewart Brand, We Are as Gods, that also covers this project. [1]
[0]https://e360.yale.edu/features/the_case_for_de-extinction_wh... [1]https://www.weareasgods.film