Man, you either need to take some basic critical thinking class, or just read an intro text-book. He is not claiming that they are secondary. It is a statement in a form '(either) A or B'.
"Skyrim can be taken EITHER as a sandbox experience - in which case its blatant shortcomings in the plot and characters departments become secondary - OR played like a traditional RPG - where the vastness of the world, sheer number of points of interest and mini diversions are all relegated to the back burner".
How Skyrim SHOULD be taken - EITHER primarily as a sand-box, with all plot shortcomings as less important that the world itself, and sand-box experience, OR a story-driven RPG, IS a matter of opinion, a matter of different values people have, and hierarchy of these values. Some people do not give a damn about the story, and only want sand-box experience, while the others, for example value both of these, but rank sand-box experience higher, or vice versa.
BTW, Bethesda people never presented Skyrim as a story-driven RPG, and in every damn interview stressed it was about the WORLD. They were building a perfect sand-box, and they nailed it. In my opinion, sand-box nature of Skyrim should be more important to the reviewers, because it was its intended major feature.
CDPR always very clearly claimed that TW3 is a story-driven RPG, and not a sand-box. So any matters related to a story-telling are more important, and sand-box features, like ability to enter every house, are less important. People can compare Skyrim and TW3, only they should do it properly, these games are not RPG twins, and have different creative philosophies behind them.