EK_Luc
For the block compatibility, Scarlet-Tech already gave you the right information 
But regarding your last question, it's not what it was intended to state. What it really means is that the terminal port where your inlet and outlet are located, are at the same place then it was on a 980 or 780Ti block. Meaning that if you upgrade your 780Ti to a 980Ti, you won't have to change your tube length or anything else in your loop except your new GPU block.
That is what I figured about the first mention about their compatibility, but then in the second to last paragraph the author states
"It is fantastic to see EK design their Titan X waterblock with a little more wiggle room than usual, with the ability to use it on your existing GTX 980 and GTX 780Ti GPUs, so users will be able to use it on their existing GPU as they save up for the upgrade or just simply want to pretend that they have a Titan X when showing off their rigs to friends or at a LAN event. "
and that confuses me. I know it is only one man's writing but it caught my attention.