The scenario assumes the startup raised ~40M from investors, which is realistic.

So in that case yes, you can own 99.9% of the equity the company, but if it sells for less than the total amount raised, the investors will exercise their liquidation preference rights (typically 1X for clean deals), meaning that they will wipe you out completely and keep the 40M for themselves.

If the startup sells for 50M, the investors can then decide to either exercise their liquidation preference rights and get their 40M (leaving you with 10M, assuming non-participating rights), or they can decide to convert their equity to common and thus get 0.1% of 50M, and you take the 99.9%. They'll take whatever is the most convenient for them, which typically means they'll exercise their liquidation preference rights unless the company sells for more than what they valued it at during their round of investment.