That's because your iPhone is like DVD quality and the HM-60x is like VHS quality, the HM-60x was designed to sound analog and smooth like CD players in the early 90's, i.e. 20 years ago.
I'm surprised the advertising hasn't covered this, I remember reading once these players were made with 20 year old sound in mind. The HM-60x don't really use the best capacitors or op-amps you can find, though, and the TDA1545 DAC chip is said to sound better than the TDA1543, so the HM-60x weren't really pushing any limits in NOS sound.
I own a DAC which uses eight TDA1543 chips in parallel, it's higher resolution than the HM-601 for sure, it's called the Lite DAC-AH, another popular one is called the TeraDak, so I can see some users like myself asking themselves, do they really need a lower resolution device with limited battery life, that won't even fit in my jeans pocket, for portable use?
Without any advertising, it seems like this could have injured the Hifiman line, so... they may opt for 2x ES9018 with something like AD797 and Nichicons now, to get a fresh start and claim a place ahead of the line. After all, we already have the HM-801 with PCM1704 and OPA627, so it makes sense in that respect.
I think they just don't see any need to advertise these facts or brief us, we're only naive customers after all, not designers right.