How I interpret that is that if one wants to do aikido without internal strength, there are plenty of aikido teachers to go to. If you want aikido with Ueshiba's kind of internal strength, it will be virtually impossible to find someone who can teach you the exact internal skills that he had. (Sure, Tohei, Shioda, etc. did aikido with internal skills, but they were different from Ueshiba's.) So you'll need to look outside of aikido for these skills. And if you really want to, you can then go and reverse engineer aikido. However, none of this is Ueshiba's aikido.
So basically what I read in the end of HIPS is: if you're interested in internal strength, why bother with aikido? Why not do what Ueshiba did, find someone who can teach you these skills and take it from there?
If you were drawn to aikido because of Ueshiba's incredible skills, it just might make more sense not to study aikido...
Hey!... that's what happened to me!
Hopefully the next generation will have the wider option of seeking Ueshiba's concepts of ai-ki within a broader availability within the art, without having to look outside the art.