That's mostly relevant for laptops & irrelevant for the iMac we're talking about, yet the MBP 16's ram type is available in sodimm's. There's also DDR4 4000 speed sodimms which match or exceed LPDDR4 at this time.Some memory technologies such as LPDDR3 and LPDDR4/X must be soldered, there’s no socketed package available.
So the option is between slow, power hungry socketed RAM, or fast, power-saving soldered RAM.