Not sure where this information is from. Parking in the city center is a nightmare for cars but bikes are fairly simple. Just pick a spot where others have left their bikes, mostly on broad sections on the sidewalk. I've never had any problems leaving my bike in the city center, there's plenty of sidewalks with plenty of space. Nobody will care as long as you don't block crossing traffic.
And by the way, paying for parking in the city center is either impossible (you have to be a resident to get a resident's parking permit) or far too expensive. We're talking 5 EUR per hour here usually combined with a 4 hour limit. Even without said limit it'd add up to roughly 3.500 EUR per month. Which is why everybody just leaves their bike/scooter on the sidewalk.
"Life is not a journey to the grave with the intention of arriving safely in one pretty and well preserved piece, but to skid across the line broadside, thoroughly used up, worn out, leaking oil, shouting GERONIMO!"