Yes, that's correct. You can declare a method like this:
[Consumes('application/x-www-form-urlencoded')]
function SumForm(A, B: double): ResultFloatType;
And XData will automatically deserialize A and B from the x-www-form-urlencoded content.
You can also use DTO classes and XData will automatically deserialize them as well, as long as the class only has fields and properties of primitive types, not structured ones (arrays/objects).