I would have to suspect the audio pre-amp in the phone being the weak spot, rather than try to blame the phone's DAC.
I would also suspect that those people who are used to a modern head unit's 4V pre-amp outputs going into their amp's input sections, are expecting a similar response from their 3.5mm headphone out, off their phone.
I don't believe this is the same thing, even if the numbers "work" and the myriad bunch of manufacturers of cell phones/amplifiers, say it is fine.
MOST of the changes in sonic quality come from the pre-amp, and not from the changes in A-D or D-A conversions, upsampling, or the like.
You can have a phone with a great digital stream capability just completely **** the bed when you use it to drive the analog out to downstream components, if the pre-amp is sucky and that's probably why you see so many people doing a bluetooth connection advertisement, with their conclusions.