That seems ... confused. AIUI, the "dark past" stuff comes up in SC review because of the possibility of being blackmailed. If it's not something that you're defensive and evasive and embarrassed about, it's not an issue. And I can't see why Musk would feel that way about his pot use.

That's not to say he's a shining example of someone you'd trust with secrets though. His impulsive tweets, where he can be easily provoked into saying very reckless things ... that seems like a much bigger strike against keeping a clearance.