Not to beat the dead horse, but I wanted to answer this question with an uber rare hypothetical:
Let's say the rule was it was the one spin and that was it, no chance at another spin-off spin. But the other player (or players) don't get the wheel around either. Then who goes to the showcases?
You can make weird convoluted rules to say someone from the other Showcase Showdown could make it, but you're now making them way too confusing.
Thus, yes, the rule as is set in place is that the contestants are allowed to spin again in that situation for a score for the spin-off. The one and done only applies to the bonuses - so if you get a dollar on the next spin, you don't get any additional money.
As for reading into what Bob says, the years have shown that he wasn't always the best at consistently describing various rules. That incident that temptation1979ga mentioned where he did DQ the contestant entirely should have never happened, and frankly the contestant should've argued to get a second chance on the show because they got snubbed hard (unfortunately with the one-and-done rule waived it's a moot point).
But at the same time, you can't exactly fault him for not remembering a rule about an event that has occurred so rarely I can count on my hands the number of times I know it's happened (the two mentioned above, the one this season from the original post, and one that happened 10 years ago, and yes, the contestant was allowed to spin again for the spin-off).