Yes, I'm also confused about this.
Existing maps should be enough, and LIDAR, etc. ensure that self-driving cars can respond to road conditions in an ad-hoc manner, instead of relying on a predefined detailed map.
The article doesn't make a good point of why this is needed. OTOH, given the money involved in the technology & companies mentioned in the article (like Here), I presume there must be a need for this.