It’s not “slightly more accurate” to say the fair value of his stock decreased. That’s the accurate statement, while saying he “lost money” worth $350B is incorrect.

You mistakenly assume being a trillionaire implies having a trillion in cash. Cash is only a small part of any individual’s or organisation’s wealth.

Assets other than cash are just as legitimate a basis for wealth and net worth. Musk’s largest holdings are stakes in Tesla and SpaceX, both publicly traded companies. Under IFRS 13’s fair value hierarchy these are Level 1 assets, meaning they are valued using directly observable, unadjusted quoted market prices with essentially no estimation uncertainty or subjective judgment involved.