As to cross training, it very well could be outdated material. Interesting point, though, because they tested with electro-conductivity, Max VO2 consumption levels, etc., fairly rigorous, concluding that not so much muscle "groups," but individual fibres their inherent characteristics (e.g., slow twitch/fast twitch percentages in a given athlete) and their trained characteristics (responsiveness to various aerobic/anaerobic/resistance stressors) matter when discussing an athlete's conditioning regimen.
It may not be outdated so much as focused on swimming. Arguably, a swimmer has little use for cycling or running as neither has a direct correlation to swimming. In the same token, a powerlifter has no need to develop a strong aerobic base, as competition is anaerobic.
I see two questions here:
1. What is the minimum physical requirements for aikido?
2. What physical attributes should aikido develop for optimum performance?
I submit that the minimum physical requirements can be met by nearly everyone and any deficiencies addressed by regular training. (Yeah, the minimum level will probably result in some bumps, bruises and injuries that could be avoided by "better than minimum" physical condition, but let's table that discussion for now)
The second question hasn't been addressed to the best of my knowledge. For example, wrestling, boxing and judo have been studied fairly extensively (benefits of having a sporting aspect of the art in the Olympics no doubt). So, there is a wealth of information available on what each art requires at the very highest levels, for example, Wayland's collection of Judo Studies for Athletes
I suspect that aikido would require a more balanced athlete than swimming/running/weightlifting for optimum performance, which would suggest a wider variety of training protocals. But then again, as Bob noted, I'm evil.