In the context of the article, if you are trying to keep foreign processes "off my cores" then you can't neglect to keep them off the adjacent hyperthreads, because those share some of the resources. If you have 8 threads on 4 cores then at least the way Linux counts them cores 0 and 4 are sharing some caches and all backend execution resources. So if you have isolated core 0 but not core 4 you might as well have not done anything at all.