I think it's very interesting that they don't use radiometric dating, like carbon dating, to establish the age of the parts of the chair on an absolute basis.
I suppose the forgeries could be constructed using extremely old wood, but even then, the glue or other parts would have to be relatively modern, and would provide a contrasting age.
Carbon Dating is typically for things at least 500 years old. While that might be useful for furniture, it wouldn't apply here.
Glue is useful - an older chair would have used hide glue, made from real animals. If a modern, non-hide glue was used that's an immediate giveaway. However, modern furniture builders still use it quite a lot in high end work today
There's other types of radiometric dating, like looking at isotope ratios to see whether it was made before or after atmospheric nuclear testing, if there are parts that would be different (typically metal pieces)
I suppose the forgeries could be constructed using extremely old wood, but even then, the glue or other parts would have to be relatively modern, and would provide a contrasting age.