The issue is the development of drugs (i.e., so-called "targeted" drug therapies, such as, trastuzumab) that depend on the result of a predictive test (HER2 protein overexpression by IHC, amplification by FISH) to meet the claims of the drug's effectiveness as approved by the FDA. As the FDA document states in the introduction,
When an appropriate scientific rationale supports such an approach, FDA encourages the development of therapeutic products that depend on the use of approved or cleared IVD companion diagnostic devices — several such IVD companion diagnostic devices for use with corresponding therapeutic products have already been approved or cleared.
As pointed out in the blog post, the implication is that the drug will not get approval unless the companion diagnostic test or methodology gets approval (and vice versa?).
You may not want to read the FDA document before operating heavy machinery or driving.