I thought it was a "no chipset" arrangement, rather than a dedicated chipset? So you stick the CPU in the bare board and all the IO hangs off the built in southbridge.
Besides, even with a "dedicated" chipset, I imagine there'll be one or two mITX boards with the full chipsets as well, just to pack a few extras in that aren't available off the SoC - maybe not the X370 (which would be rather wasted), but the B350 wouldn't surprise me at all...