In fact, its even the opposite :
the current RC already has an official YUM repository with RPM packages.

It's just that you have a chicken and eggs problem :
Before having Sailfish X on your Xperia X, you don't have pkcon (or zypper, depending on your preferences) to install the RPMs to actually install Xperia X.
(And same goes for the other tools like lvm which are necessary to create the partition-scheme used by Sailfish X).

So it's much simpler to straight flash a ready-to-use image the first time and only use the YUM RPM repositories for upgrades or getting software that isn't installed by default.

(Rather than going through the manual route, which will probably require telneting into hybris-recovery.img image, copying extra stuff over netcat, manually partitionning, manually build initial RPM database, setting up reverse tether and install all the RPMs one-by-one).