The 20% gain is 100% a consequence of the Fed and the market mania. This experiment attempted in another 5-year span would have worked out totally differently.

The largest pitfall is definitely housing. He mentions not wanting to own a home. He also must accept that rent rises faster than inflation, and he may have to move every few years to cheaper and cheaper places to keep things balanced.

If you can’t guarantee you’ll be able to stay in your community, with your friends and family, for the rest of you days, what’s the point? Moving sucks. Not being able to eliminate moving is a huge lifestyle and stress liability.

OP addresses this. He points out that when he retired, one of his fears was that his nest egg was propped up by market mania. But since then his nest egg has increased. Sure, you can say it might have worked out differently in different time periods, but that's not necessarily a problem. The point of making these models before retiring is to also take into account the pessimistic conditions, and it appears he did so, and judged it all still possible.

He barely kept up with historical inflation rates (3%/year). Given the free availability of money in the last 2-3 years, it's hard to say a 20% appreciation on his capital is a success. In terms of real dollar value, he likely is losing year-over-year.

No, he says he has 20% more after adjusting for inflation. He has gained 20% in real dollar value. Even if that's only ~3% per year, that's fantastic on no (traditional) income, after accounting for expenses.

Regardless, even if he was break-even or negative, that might be ok; most people don't expect their retirement savings to last forever, just until they die.

Are you accounting for his spend or only looking at beginning and ending balances? When you retire early, you don't assume you'll beat inflation while also spending some of your principal.