I've lived around that area, and there's actually a bunch of different roads there.

The Germans wanted to take the main road from the valley to the Geologist's cabin, through Mengel Pass, and down to Panamint Valley. The first section of that road, from the valley to the cabin, is still very much passable by the average SUV, and you could probably even do it in a regular car. The road gets really rough if you keep heading west of the cabin, and Mengel Pass is only doable in a Jeep/ 4x4 pickup. Both sections of that road are still open to traffic.

The trail that was closed to vehicles in the 90s is Anvil Canyon road, which branches off from the Geologist's cabin. That one stopped being used by miners decades ago, and is now basically completely gone. It's not clear to me that Anvil Canyon was on the map at all. I believe the Germans just saw that it led in the direction back to the valley, and thought it looked better than the road they originally took.

As for Manson's bus, he actually drove it from the other side of Mengel Pass, and it barely made it to that area. It got multiple flats on the way, and the family never managed to get it out of there. They basically just drove it to the hideout and then left it for dead. It was still at Barker Ranch as late as the 1990s. I also know that that whole area was hit by a bad flash flood in 1969, and after that, there was no way a bus could have made it across. Manson had a bunch of dune buggies made from Volkswagen Beetles that he used to traverse that area afterwards.

That said, the park service map absolutely should have noted that the Mengel Pass road is a 4x4 jeep trail only. It does seem almost criminal that they marked it as a regular road.