The highest density hard drives have 10 platters[1], storing about 2.2TB each. So if they're talking density in terms of bytes per square cm, 24 is probably reasonable. HDD platters are certainly smaller than DVD-size, and the 24x claim would be true if they were ~25% the size of a DVD which seems about right.

[1] https://blocksandfiles.com/2023/04/13/seagate-22tb-disk-capa...