A good bet when assessing a metallurgical statement in the framebuilding field is that it is wrong. Almost all the collected wisdom, particularly that comparing different materials, is wrong. Aluminum is not a popular material among frame builders so most statements are not only incorrect but also biased.

For instance, rims operate in compression to the extent that you can cut them into multiple segments and so long as you can hold them in place while the wheel is built, they will be strong without anything other than the spokes holding the parts together. This doesn't really tell us anything about the way a frame works.

I'd use an aluminum frame and not worry about it. I do prefer steel forks for touring, but for the rest of it aluminum is fine. Works for climbing gear, one doesn't hear this overwrought nonsense about karabiners, or cams. I do prefer steel frames because they are easier to make in my shop, and I don't see any real downside to steel. Same characteristics make steel more easily repaired though, if one knows how to go about it aluminum can be repaired pretty easily in many cases also.

The reason you don't hear about climbing gear failing is because it's not subject to the same sort of repeated, cyclic loading that bicycle parts are... Plus, bicycles just get a lot more usage that climbing gear. Bicycle tourists ride for hundreds or thousands of miles each year. How many climbers do you know who climb millions of feet (1,000mi = 5,280,000 feet) per year?

In all honesty, the type of material isn't nearly as important as the quality of the construction. The relative strengths and weaknesses of any material is taken into account in the design of the frame using that material.