I think the difference is that there is still a maintenance of stored value, but that stored value is now able to earn more yield than what you would by depositing it at a bank.
In both the cases described in the article - using Unit or Bridge - you pass on the work of stored value to someone else. But Unit doesn't earn as much by being the stored value as Bridge would because Bridge is invested into T bills / MMFs. Hence, Mercury coin is better than running a bank using Unit.
Is that fair to say?