You would need a separate cable.
2.5mm balanced advantages:
+Can be adapted to other balanced or single ended terminations
-Pinout (which “ring” section of the barrel connects to what) isn’t standardized, so it will work with the cables sold on Drop, but not all cables from other sources
- Thinner, smaller plug than standard, more easy to break (I used to see broken 3.5mm plugs in phones, this is smaller)
This Drop adapter isn’t bad itself, and might be good if someone has an IEM with a cable they bought on drop, but for an HD 6XX it’s easier to just use the stock cable (3.5 plus 6.5mm adapter) and get an XLR cable for balanced (and maybe ponytail style adapters from XLR to anything else you need, IF you ever need balanced besides XLR).