My friends, thanks a lot for your help. The "full" modification was also what I had devised. Or to use a cell array to store the sparse list. But to me it was important to understand why. And you pointed me to the techdoc page with the release note.
Also thank you for the "permute" idea!
By the way, yes, I believe this apparently meaningless copy is to "extract" a small matrix from a huge one. Out of its context, it does seem odd. But it isn't.