The D2 or S9 cannot even drive 16Ω loads without falling flat other than volume, so these spec are silly. Just like the low-fi world where manufacturers advertise based on output spec, these DAP are doing the same. The problem is that they rarely, if at all, meet any of their own requirements.
Almost any DAP will get loud even for headphones like the DT880. My touch, AMP3, even fuze get pretty loud with them. But few can put the same finesse and power into smaller channels like 16Ω earphones, or balanced armatures which oscillate in impedance.
Just like the post ^^, it is about driving headphones, not about volume. That, however, seems to be forgotten as manufacturers put better and better spec indicating unloaded outputs which never ever live up to their numbers.