Hola Hamish

When we refer to food as being tasty we use "está", está rico, está delicioso, está salado, está soso...'; this is what we are conveying here in that sentence. We may use sometimes "es delicioso" to refer to food but in that case we imply that the "quality" of that food is really good, so it is a more "rounded" meaning, not only the taste but also the quality in general, as if being delicious was part of the essence of that food. 

Saludos

Inma