In the section on choosing a frame by material construction, the author makes a claim that all Aluminum frames will eventually start to crack and fail with normal use (accelerate that demise with extra loading, hard riding etc). He also says that steel will last forever, providing it doesn't rust.
"Sadly, an aluminum bike will always be mortal"
So can be steel though! I've gone through three steel frames in the last decade, unfortunately.
I think the statements he makes here are very generalized.