Superimposing a landscape page with 90 degree rotation onto a landscape page with 0 rotation

(As you referenced a Java example, I'll also refer to iText for Java.)

In the SuperImpose example the pages to superimpose are added using

canvas.addXObject(page, 0, 0);

but there are other PdfCanvas.addXObject overloads, too, in particular

public PdfCanvas addXObject(PdfXObject xObject, float a, float b, float c, float d, float e, float f)

The 6 floats represent an affine transformation applied to the XObject, e.g. a rotation. Thus, you can rotate the imported page XObject from the source document to match the orientation of the destination document page, e.g. replace

canvas.addXObject(page, 0, 0);

by

canvas.addXObject(page, 0, 1, -1, 0, page.getHeight(), 0);

or, if that makes the page contents stand upside down, by

canvas.addXObject(page, 0, -1, 1, 0, 0, page.getWidth());

Just like the example you referenced I here assume the lower left corner of the pages involved are the respective coordinate system origin. If that assumption is wrong, one needs to adapt the last two float parameters respectively in the replacements above.