That's because people on Hacker News don't know shit. Everyone here who has a superficial understanding of deep learning and believes they know what an MRI is comes up with extremely strong opinions, but it is very obvious that noone here has even read the full article beyond the abstract (or even the title). There are legitimate questions and concerns you can raise about this article, but not a single one of them is found in this comment section.
This paper is actually the culmination of a series of developments in the field over the past decade. Anyone who was following the subject was not surprised by it. Yours and all the other comments here are nothing but a testimony to the presumptuousness of HN. It's also funny how posts about modern physics usually just yield admissions that people know nothing of the subject, but when the topic is AI or medicine, everyone is suddenly a research scientist.