I saw a photo of a park service person holding the photo up with a building at Arlington in the background and you could see that it was clearly the same building as in the photo. It is distinctive. So perhaps they are not Lee's slaves but if not there are only a couple of other possibilities: they are slaves from before Lee took over the property, they were freed slaves shortly after the war (which is almost certainly the same as saying "Lee's slaves") or they are from the slave period and are on Arlington but they weren't Lee's. or it's a modern fake
Personally I think the most likely scenario of a photo of slaves on Lee's estate and marked "Lee's slaves" is that they are indeed Lee's slaves.
or maybe they were tourists after the war for a photo?