The Shz article is outdated but doesn't state
that you may not let your dog run free in Schleswig-Holstein.
It merely outlines the specific legal situations when it is not allowed to let your dog run free. So, the generic legal rule is that you don't need to keep your dog on a leash until there is a legal provision requiring you to do so.
There was a trend to have general restrictive legal rules for dogs that were presumed to be dangerous a few years ago but several courts rightly pointed out that all such rules must be proportional, so couldn't impose too strict generalized, abstract rules without taking into account if a specific dog is actually dangerous or not.
In Schleswig-Holstein, a new dog act ("Gesetz über das Halten von Hunden (HundeG)" entered into force in January 2016. It contains differentiated rules, especially in its § 3, on when dogs are to be kept on a leash and when this isn't supposed to be required. In a nutshell, a leash is required in places where typically many people are, like public parks or pedestrian zones. This it not much away from the previous rules.
For walking dogs in a forest, there is still a special rule (which has precedence because of the lex specialis principle) in § 17 of the forest act of Schleswig-Holstein of 2004 ("Waldgesetz für das Land Schleswig-Holstein
(Landeswaldgesetz - LWaldG)") according to which dogs in the forest are to be kept on a leash at all times. The reason behind this rule is probably that freely running dogs in a forest may easily hunt wild animals living in the forest.
If you don't obey these rules, you are typically committing an administrative offense ("Ordnungswidrigkeit"), so you could end up with a fine.