Earlier quoted context omitted.
The other property of aluminum that is relevant for some uses (but likely not ladders for most uses) is that, unlike steel, it doesn't have a well-defined fatigue limit. Therefore, it can eventually fail after a large number of even relatively small loads that don't visibly deform the material. That's one of the main service limits on airframes.
Definitely true. Steel will stretch or bend if overloaded but aluminum is brittle enough that it will break once but without any warning of fatigue that may have occurred. Now, in this case I wouldn't want to use steel, because it rusts and is heavier and neither are good for fire fighting.
There's a cyclist saying: steel bends, aluminium breaks, and carbon fibre shatters.