The 3.5mm audio jack has 4 wires in a TRRS layout. This can be used for a separated ground output. Each channel has a dedicated Grd ( R+/R- and L+/L-) instead of the Common Grd in the normal 3 wire TRS configuration (R+/L+/R&L GRD) HiFiMan(and some other companies) supports the same TRRS separated Grd output.
The Sony DAPs, ZX, WM series, all have this TRRS capability on the 3.5mm. This provides a separate ground for each channel. This is not true balanced, but sounds better than just TRS. The Sony automatically detects the TRRS jack and switches to separated ground.
The normal 4 contact TRRS 3.5mm found on phones, uses TRS for audio, and uses the 4th contact for the microphone.
The Sony DAP (and most others) do not have a microphone. Only the older model (much older) DAPs had combined earphone remote controls. These were usually all in one, and proprietory. Some models implemented a short dongle with the microphone, and a female 3.5mm jack to plug a separate earphone into.
With todays market demands, there is no market for a wired dongle with remote, to plug into a DAP, and then plug a multiple $K IEM with special cables into this dongle, with unknown wiring and the extra complexity of an added circuit board and contacts, with the potential sound deterioration this could cause. Thus this capability is not built into DAPs. And it is highly doubtful that they ever will be.
Sony DAPs will only work with the small Sony remote. Why does it not work with a generic BT remote is a good question. Unfortunately, there is no answer....