As an indication: use Google to search for images of Holz (or any other German noun. This link accomplishes that.
The results are a strong indicator of how most people use a particular word. In this case, Holz is most closely associated with timber, lumber, chopped wood, wood products, etc. More the results of taking down trees than a stand of trees.
Yeah, I'm pretty sure.. but not 100%, since I didn't take a screenshot. Anyhow I clearly remember being surprised.. also because, if I made a mistake using the word Wood, it should have correct me on grammar. Instead, the suggestion was about using Timber. That sounded weird.. :)
In this same exercise, Duolingo gives "Das Leder" and if you write "leather" it is marked wrong. Then here for "Das Holz" I wrote "wood" and it is marked correct. What makes "Das Leder" = "The" leather where "Das Holz" is accepted as plain "wood"? I know Holz could mean wood, timber, lumber. When is "der, die, das" necessary for "the" and when is it not? Does Die Baumwoller = "The cotton" or "cotton"? Leather, wood, cotton, milk, water are all uncountable in the generic sense.
Lumber (American English; used only in North America) or
timber (used in the rest of the English-speaking world) is a type of wood that has been processed into beams and planks, a stage in the process of wood production. Lumber is mainly used for structural purposes but has many other uses as well.