You might have to get on the ground in some cases to know if what you are seeing on lidar is actually a kiln. But in the case of the kiln clusters in areas such as that around Hanover Furnace, you can be confident that you are in fact seeing kilns and not vegetation.
I made it out to where I thought the mill site might be. I left disappointed. The area is thick with briars and I wasn't able to get a good look. I saw no evidence of a dam or race, but I can't claim to have done an exhaustive search.
And you were driving on Carranza Rd (not Hakwins Bridge Rd, btw)? Look at the target: https://online.boydsmaps.com/#17/39.75013/-74.58487/pines2020
If so, the ranger is either inept or full of sh*t. Either way, I'd contact the Park Police about it.