Your suggestion to implement a "clearly defined taxonomy" for decision-making is an attempt to impose rigor, but it potentially sidesteps the more pressing issue: how these LLMs are likely to be used in real-world, less structured environments. The study seems to simulate a plausible scenario - an HR employee, perhaps unfamiliar with the technical specifics of a role or a CV, using an LLM with a general prompt like "find the best candidate." This is where the danger of inherent, unacknowledged biases becomes most acute.
I'm also skeptical that simply overlaying a taxonomy would fully counteract these underlying biases. The research indicates fairly pervasive tendencies - such as the gender preference or the significant positional bias. It's quite possible these systemic leanings would still find ways to influence the outcome, even within a more structured framework. Such measures might only serve to obfuscate the bias, making it less apparent but not necessarily less impactful.