The number is quoted from being averaged over several years. I think the basic premise is correct though, one can live off that type of money passively and comfortably (depending on where you live of course). If you aren't seeing those kind of returns on average, move your money to a total stock market index fund.
The premise is incorrect because real rate of return is what determines how you can “live”. Even if you received 10% returns every year, it makes a huge difference if the medicine/food/labor you need increased by 5%, or 10%, or even 20%.
Additionally, if you have lost the ability to earn an income, then you cannot afford to be down a few years, hence you need to greatly expand the proportion of assets that are earning less than equities, and quite possibly putting you behind inflation. Especially if you live in a popular area.
Combine this with a nation whose population is aging and therefore competition for buying young people’s labor is going up, and you might want to be more conservative about expectations of the future.
Better plans is to plan on living off 4% of you nest egg if you don't want to run out when you are too old to start working again.