The --rehash flag of the db-in recipe seems to have no effect. The reason seems to be that the example with the new hashes is assigned to the task variable but this variable is never used:
if rehash:
task = set_hashes(task, overwrite=True)
Thanks for the report, I think you're right and this might be a leftover from back when set_hashes would modify examples in place. We'll fix this for the next release! (In the meantime, you can just edit the logic in the recipe yourself within your Prodigy installation.)