I know. From the linked article: "The simulation showed that wurtzide boron nitride would withstand 18 per cent more stress than diamond, and lonsdaleite 58 per cent more." The title of the article is misleading.
The title of that article is accurate, where it says "stress" they are really referring to hardness and not tensile strength. The quoted 58% comes from this paper, it talks about "hardness" and "indentation strength" in the abstract:
You are right, I stand corrected. They actually talk about tensile strength in the PRL article:
"It is noted that lonsdaleite exhibits almost identical ideal tensile strength and only slightly larger pure ideal shear strength compared to diamond. The significant enhancement in its indentation strength occurs under biaxial stress loading conditions. The situation in w-BN versus c-BN is similar. All past calculations have shown that diamond exhibits the highest strength under various loading conditions compared to other materials, which was consistent with all available measurements. Here we show for the first time that w-BN and lonsdaleite exhibit higher strength than diamond under indentation."