They assume a retiree puts 60% of their portfolio in bonds, and note how bonds are at historically low rates right now.
IMO that's a run-for-the-hills kind of conservatism. Using numbers to illustrate, suppose you want $40k/year in spending post retirement. They advocate a $1.43MM with more than $857k held in bonds. Assuming the bonds just match inflation that's equivalent to a 21 year bond ladder.
...sound nuts to you? because it does to me...