Transcendent greatness in sports is both absolute and relative.
Absolute, because anyone who sees an exceptional performance can recognize it as exceptional judged against nothing but its own merits.
Relative, because we seek to define greatness by comparison, to determine levels of it (greater? greatest?) when judged by other exceptional performances we have seen or know of, no matter how hard it is to make such comparisons across long periods of time, with the wildly different athletic parameters of different eras.
No matter which standard we use, absolute or relative, what Nathan Chen did in winning the 2019 World Figure Skating Championships in Japan was transcendent greatness.