what 'rosy provisos'? He was analyzing 30-year retirement portfolio returns starting at 1926 and came up with 7% average SWR.
Assuming our retiree is lookiing ahead, not behind, and he consider a SWR as something higher than 50% success, then taking 7% each year requires two handy optimistic provisos: good portfolio returns on the early years and no worse than moderate inflation. And the unstated one: that the historic returns of the world's most productive economy are a good data set for predicting results over the next few decades.