Hi we stayed there a few years ago. It's not a very big site but we thought it was lovely. I don't see why tents couldn't go but I can't remember seeing any when we were there.
It has toilets, shower, washing machine and the staff at the time were really nice. There was a number of seasonal caravans on the site and I'm not sure if you can book up in advance.
I've had a look on their website and they do give prices for tents (Zelt). They don't really have pictures of tent campers though, which might mean tent campers are not really on their radar.
I think it is possible for some sites to focus a bit too much on tin tenters and a couple of times we've found ourselves on manky pitches at the back of the site, or pitches churned up by wheels while the caravanners are all on lovely soft grass. Or stuck next to a caravan listening to doors slamming and feet thundering seemingly all night. To me that's quite a biggie and I pick my sites accordingly.
The reviews for Sonnenstrand look pretty good. Clean and friendly but you do hear the boats and trains as they pass.
But the answer to your question is yes they do take tents :-)