The fact that the author presumably believes more (it's not that clear what they do believe, which is a weakness) doesn't make what they do directly advocate "not a position".
In that case what does he mean by "Having prisons is undesirable"?
It's possible that they would agree that the immediate outcome of abolishing prisons would be bad (in the same way that the immediate outcome of abolishing slavery hurt some people) and they might even agree that they shouldn't be abolished all at once. It's not clear that they wouldn't agree, anyway.
The actual statement just means that a utopia wouldn't have prisons.
Another month, another rationality quotes thread. The rules are: