Hi: The statement that: "Pre-hire assessments should never be the only determining factor in your hiring decisions. Not only are you passing over qualified people, but your actions might be interpreted as discriminatory" is quite misguided.
First, while employers certainly do not make determinations to hire on the basis of one factor (e.g., assessment, inteview, background check), they generally use a multiple-hurdle process, whereby applicants don't progress to the next step in the hiring process if they are not successful at the current step. Minimum qualifications, assessments, consumer reports, initial interviews, interviews by hiring managers and drug testing are common hurdles, which applicants must clear--rejection will occur due to performing poorly on any one step. From administrative, operational and cost perspectives, employers are not able to have all applicants participate in every step of the hiring process.
Parenthetically, the employees of the person posing the question would have performed adequately on all hiring measures in order to be hired--hence, the focus of the question should have focused all the selection measures that lead to hiring a low performing employee. Bottom line, an employer should not be placing reliance on any hiring tool that is not job-related and consistent with business necessity. However, even using best practices, there will be hiring decisions that are incorrect.
Finally, using pre-employment assessments to reject applicants is no more likely to be viewed as discriminatory than using criminal background checks, interviews, drug tests, medical tests, etc. a sole basis of rejection. In fact, an objective (pass-fail) application of a test or other hiring tool is the most defensible position. Further, with respect to assessments, excluding cogntive assessments, there is extensive research evidence documenting that they typically do not exhibit disparate impact.