At the age of 18: https://en.wikipedia.org/wiki/Viswanathan_Anand#Early_chess_...
Also, according 2700chess.com he was #96 in the world with a rating of 2520 on January 1988. That's right after turning 18. Growing up in India (before the chess boom). Before chess engines existed. 6 months later he was #49 in the world. https://2700chess.com/players/anand
A year earlier (at 17), he had won the world junior chess championship (U-20). Not exactly a "strange meteoric rise" for Anand. The "GM at 18".
https://en.wikipedia.org/wiki/World_Junior_Chess_Championshi...
Meanwhile, at the same age of 18 (already during his covid boost), on July of 2021, Hans Niemann was #20 among all juniors (U-20), with a rating of 2571. Close to #300 on the global rating list: https://ratings.fide.com/profile/2093596/top https://ratings.fide.com/profile/2000059
Unlike Anand, no achievements of note until then (ok, he was 9th at U-16 World Youth Chess Championships in 2019). A year later he's a top 40 player, hanging out with the best and beating the (incredibly dominating) world champion with the black pieces. Make it make sense.
Also, take a look at this analysis of correlation between average (and stddev) of move accuracy vs rating:
https://www.youtube.com/watch?v=YnnJ0Da4Rp0 - explained in the detail
https://www.youtube.com/watch?v=Q5nEFaRdwZY - analysis applied to several players