I can't speak from personal experience with the Fiio jm21, but I was a big user of a previous generation of Fiio, and while I imagine some technical leaps forward have been achieved with this generation (the Fiio M1 never, for instance, achieved gapless playback from 2015-2021, even though this was promised with every new software version), taking a quick look at it... this is just an android phone interface! App store? Chrome? I certainly don't want this from a dedicated music device

Beyond this, I'd say that the true advantage of the iPod Classic was a matter of polish and UX:

* Dedicated buttons/wheel/etc that are tactile instead of a touchscreen interface (the Fiio M1 was button-and-wheel based, but it never approached the quality of Apple engineering); I see the jm21 has some side-based buttons for pause/forward/back, which is nice, but a touchscreen as main interface still grates * A way to interface with your albums that was delightful and visually dense (Cover Flow remains the single greatest music UI put forward)