Bad data - the report used fuzzy public videos of DoD intercepts to determine whether warheads were impacted. However, the very limited publically available data contains no way to know the actualy impact point or, most crucially, the impact's effect on the warhead.
Bad assumptions - the authors assume that the warhead would continue onto their original targets if not directly impacted. In reality, the hypersonic velocity impact of the interceptor on the missile body has the effect of a very energetic conventional explosion very close to the warhead. Very large impact forces would be transferred to the warhead through the missile body and also via fragments produced in the explosion. There is zero probability that the missile warhead would continue on its original course. There is high probability that the warhead would be disabled or destroyed by these forces, which will be much larger than any forces experience in reentry.
The authors contend that 9 out of 10 intercept tests could not be considered successful. In fact, in all likelihood they were highly successful and verifiably so via sensor data of the debris field after impact.
In other words, the authors of the study put out garbage masquerading as analysis.