Compared to what we've done in the last 50 years they were rank amateurs.

They were pretty thorough at wiping out entire classes of large land animals in North America (and probably everywhere else but Africa and parts of India), at least if the theories hold.

The iron age required a lot of wood.

The iron age was a long time ago, America had not been 'discovered' yet.

Just two pictures to give you an idea of what has been lost:

https://i.pinimg.com/550x/73/62/26/736226b8437e6ce9eb1bab42e...

https://pbs.twimg.com/media/B7a4mTiIIAEsESF?format=jpg

Multiply by a few million or so...

England used to be covered in dense forests of oak and other trees. It got almost completely deforested to build ships, among other things, and many of the areas never grew back. And that is what we know from detailed records. Many of the other changes there are no records. We don’t know full details of how extensive the megafauna was in the America’s before humans showed up, and how extensive and intentional was the hunting to extinction of them was. But it definitely happened and at huge scale for a long time.

We have evidence of wide scale changes made by humans long before written records existed. We also now have better records, so it’s easier to point to this one here done by this person, instead of only picking up what is visible in the fossil record (which is by it’s nature spotty and incomplete). But that doesn’t mean it didn’t happen.