Are normative and ipsative opposites?
No. This is a common understanding. Normative and ipsative are not opposites.
Research has found that forced-choice (ipsative) assessments, like Plum's, correlate around .70 with likert scale assessments (normative) across a range of traits. Meaning they have ipsative and normative assessments have an exceptionally strong relationship. This also means that the idea that within-person judgments tell us nothing about between-person trait elevations is false. In other words, if a person is higher on Trait A than B, C, and D, then it significantly increases the odds they are higher than average on A in the population, too.