that post is wrong and should be deleted or corrected.
It wrongly quotes dimensions from here:
http://line6.com/support/page/kb/_/pod/pod-hd/pod-hd500xhd500pod-hdpod-hd-pro-faq-r26
these:
Height: 2.91 inches (2U)
Width: 21.5 inches
Depth: 10.29 inches
are (ofc not rack size) HD500 dimensions and not hd PRO.
Anyway i found a Pro X owner and for the record the thing is 19cm deep