The interesting part is that the 13 years sentence might actually be illegal, and won’t hold up against an EU court.
As Puigdemont was in Schleswig-Holstein in Germany at the moment of the arrest, German law limits the crimes for which Spain could extradite him, and limits the maximum sentence Spain can enforce, per the extradition agreement. This also applies to the cases which happened after the extradition.
One can speculate why he specifically chose Schleswig-Holstein, but it’s the only region in Europe where, 1920, due to a peaceful referendum, a part of the country separated, and joined another country – the Northern Schleswig seceding from Germany and joining Denmark.
Which is why the judges in Germany ruling over the extradition were likely more positive towards him, and first ruled that any and all cases against him for treason, secession, and anything like it have to be dropped, and he can only be extradited to spain under the condition that spain only prosecute him and any people he worked together with for misuse of public funds.
And this ruling by the court deciding over the extradition also meant that spain can at most enforce a sentence of a few months, but mostly only a monetary fine.
So what Spain is doing here is legally against most EU laws.
Which is not the first time, considering EU law specifically demands the right for self-determination for any and all people, minorities, and regional groups, such as the danish minority in Schleswig-Holstein for example, and the same applies to the catalan minority in Spain.