Six-trillion tree claim
Fragment of a discussion from Comments:Study estimates Earth has over three trillion trees
It's the big question I had writing it, too. It feels like an impossibility to me, to have any real concept of what populations were 12,000 odd years back. Nonetheless, that's the figure they offered.
All in all it seems straight-up odd also that multiple studies have apparently, presumably independently, come up with the 400 billion figure. There's a lot going on here that I'd have loved to have had the time to dig deeper into.