My guess is syrup based is referring to high fructose corn syrup, which seems to be the base of many fizzy drinks. I think this syrup is mixed with all the drink's other flavours, then carbonated water is added.
This isn't a statement of fact, just that I know coca cola (amongst many others) uses the stuff.
On a related note- when I was in Canada, visiting my cousin, I got the taste for Mountain Dew. It's one of the few fizzy drinks that I really like the taste of. You can't get it here. It was marketed for a while, but was withdrawn due to poor sales.
I'm not a big fizzy drink fan anyway, so wouldn't drink much of the stuff, but it would be nice if now and then I could find a can! There are a couple of sites on the net that import it, as well as other US brands, but they really know how to charge! One day I'll order a Hershey bar, just to try it - another thing you can't get here!