Undeveloped land - definition

There is no time frame in regards to the previous development and your site has previously been altered by direct human activity. So I would define it as previously developed.
The intent of the credit is to avoid inappropriate site development. So as long as you meet the other criteria, I think you are following the intent.
I actually had a project on a urban site, which also had new tree development since the previous buildings were destroyed during the 2. World War. The local regulations required as to protect some trees but other than that the site was consider as previously developed.

USGBC has more specifically defined "previously developed" in its Nov. 2011 addenda. This is very helpful in terms of clarifying that some land alteration and human hands, like agriculture, is NOT considered development. Here is the definition:
Previously developed sites are those altered by paving, construction, and/or land use that would typically have required regulatory permitting to have been initiated (alterations may exist now or in the past). Previously developed land includes a platted lot on which a building was constructed if the lot is no more than 1 acre; previous development on lots larger than 1 acre is defined as the development footprint and land alterations associated with the footprint. Land that is not previously developed and altered landscapes resulting
from current or historical clearing or filling, agricultural or forestry
use, or preserved natural area use are considered undeveloped land. The date of previous development permit issuance constitutes the date of previous development, but permit issuance in itself does not constitute previous development."