I think by "hard" he means "might not achieve it". But they actually already exist, so they're automatically possible.

The existing systems could not be defined as self-driving. And the idea that we could have had them in the 90s is ludicrous.

We did have them in the 90s:

http://people.idsia.ch/~juergen/robotcars.html

http://www.roboticstrends.com/article/back_to_the_future_aut...

You seem to have a personal definition of self-driving. What I was saying was to have self driving cars, today you need to:

a) resolve the social issues around self driving cars.

b) install a small amount of infrastructure to enable self-driving cars/standardize roads.