And you're on the money about the backside. He was trying to emphasize the exaggeration of the spreading of the fingers though. Saying that unnecessary localized tension is a nono.
Funny thing is, there's a lot of ...Shihan out there (I won't say what style) in magazines like Hiden showing them tensing the small of the back and spreading the fingers as an example of "extending ki"
Of course, everything is open to debate, but I'd forward that this kind of bodymechanic would only slow down transmission...even though it might feel strong initially.
One of the levels of Tohei's unbendable arm has the tester placing pressure on the fingertips specifically for this point also. Any tension in your fingertips and you'll either fumble the test or your fingers collapse when the test is applied.