Zero notebook will use it. Practically DDR4-2133 and DDR4-2400 SO-DIMM is available in mass for a reasonable price, in a year DDR4-2666 SO-DIMM might be feasible but that's it. And even this might be too high because Kabylake goes up to DDR4-2400, means demand for higher SO-DIMM DDR4 is limited.
Even dual-channel DDR4-2400 SO-DIMM would be 38.4 GB/s, a little bit more than a 1/3 Polaris 11 bandwidth. And it's sharing this amount (and the TDP) with 4 Zen cores.
If there really is a DDR4-3200 out of the box support, then it is more likely referred to desktop where such modules are cheap and available. Although I'm sceptical, it could be also the maximum with OC.