Sacks’s statement is explicitly conditional: if the South Korean LK‑99 claim were real (valid and replicable), then it would end up being regarded as the most important discovery in physics of the 21st century.

Follow‑up work in 2023–2024 showed that LK‑99 is not a room‑temperature, ambient‑pressure superconductor. Multiple experimental and theoretical studies found it to be an insulator or, at best, a non‑superconducting correlated material, and replication attempts failed to show zero resistance or a Meissner effect.(techcrunch.com) Reviews in Nature and other outlets now treat the original claim as refuted.(researchgate.net) As of late 2025, there is still no accepted room‑temperature, ambient‑pressure superconductor.(theguardian.com)

Because the condition (“their result is real and replicable”) turned out to be false, the world in which Sacks’s forecast would be tested never occurred. We therefore cannot say whether, had LK‑99 been genuine, it actually would have become regarded as the most important physics discovery of the century. The prediction’s truth value cannot be determined from real‑world outcomes, so it is best classified as ambiguous rather than right or wrong.