He is probably on here under a different name just like many celebrities are in video games using alt names. I suspect he would only use his real name with proof if he felt this was an audience for him. Most of the HN audience will expect references and afaik Elon just posts his opinions and not references, so this is not likely a receptive audience.
1. Little to gain for most tech CEOs, and a lot to lose; a much less dramatic environment than twitter, but one where missteps or embarrassment could have more serious consequences.
2. I wish Twitter would allow sorting of replies (by likes or whatever).