It's his repeated blatant disconnection from reality (a.k.a. technically not lying, but...) in his remarks regarding production levels of the Model 3, regarding

It's his repeated blatant disconnection from reality (a.k.a. technically not lying, but...) in his remarks regarding production levels of the Model 3, regarding Tesla's financial state, and most importantly, regarding the capabilities of "autopilot" and refusal to take responsibility for technical failures in said "autopilot." I wouldn't say HN is mad at him so much as wanting him to be responsible and own up to reality. He's supposed to be an engineer, and Proper engineers don't engage in "spin." Furthermore, "spin" has to be somewhat believable, whereas Musk has had a recent history of blatant falsehoods. Just like HN calls out anyone else when they're wrong (that is, disconnected from reality), HN is calling out Musk now that he's verbally disconnected from reality. HN has a low BS threshold.