Does not state whether the do or do not
Whether the company uses performance indicators is not clear, but at least the modern slaver statement does not define them. So, the answer should be "no," rather than "unknown."
pg. 1
“OUR EFFECTIVENESS IN ELIMINATING MODERN SLAVERY
We will periodically review the effectiveness of the measures we take in ensuring that there is no modern slavery in our supply chain, and we will use the results of that review to inform the development of our procedures and best practices.”