That's often true when you're talking about a single project with a clearly delineated end state, such as, "Build a responsive website for my coffee shop that works in all major desktop and mobile browsers."

But it's generally not true when you're talking about advances in technology and market dynamics where there are multiple inflection points. If I told you that the products Apple is planning to build over the next 3 years are going to be harder to build than anything it's built in the previous 45, simply because the last 5% of anything is the hardest, you'd probably say, "That's not what I mean!"

I think it's pretty obvious that building autonomous vehicles that can understand human driving behavior without direct communication is harder than building autonomous vehicles that only have to worry about other autonomous vehicles, with which they can communicate in data-rich ways. The fact that the former needs to happen before the latter because of market dynamics doesn't change the fact that it's a harder technological problem.