Zen is not due out until at least a year from now. Until then, the current and new CPU's will continue to use the current motherboards.
But starting with Zen, all new AMD CPU's will use Socket AM4. It will not matter if the are just a CPU or if they are a SoC. They will all use that new socket and motherboard. So what AMD is doing it making it so that no matter which motherboard you buy starting with Zen, any AMD CPU will work with that one socket. And this allows people to buy a CPU this year, and next year to buy a SoC, and use both on that one motherboard. There are also going to be some other changes with Zen that those new motherboards will facilitate. So they did have to change motherboards anyway, and this was a smart time to...