DRAM controllers used to be on the motherboard, not the CPU. That changed for AMD in 2003 and Intel in 2008, but it is not at all interwoven with the CPU core itself. Processors within the same product line with varying core counts use the exact same DRAM controller blocks. This is really obvious on the Intel server chips where the CPU cores are linked by a ring or mesh bus and the DRAM controllers are just another stop at the far edges of those interconnects.

Intel can update their DRAM controller independently of the CPU core microarchitecture, just like they can update the integrated GPU microarchitecture independently of the CPU core microarchitecture. The only reason why they might not be releasing a CPU with LPDDR4 yet is if they never expected to need an LPDDR4 controller before their 10nm process was ready, and never started designing a 14nm LPDDR4-capable controller. If so, that's a clear miscalculation on their part and a sign that the processor architects are probably insufficiently skeptical of what the fab guys are telling them.