The calculation doesn't seem so weird. At least he shows that the current price of ~750 implies a yearly return rate of ~50 of the price of 1 BTC goes to 2.5M in 2033 (2.5*10^6 / 1.50^20 = 752). It's good for people to see that the undervaluation (although I believe still exists and is huge) is still a lot less than t has been in the past. A early ROI of 50% is far lower than we have seen in the past.

BTW: A more normal return of 10% (which is common in valuing companies) would imply a value today of $371609. But I believe a far higher discount rate is warranted in the case of Bitcoin.