From Anandtech's article:
It was stated in our briefing call that there will be a launch of a future Zen2 APU for the consumer market compatible with 500-series motherboards. The company specifically did not say 400-series, but did clarify that the 4000G series announced today was for 400 and 500 series.
So these chips will work on both 400 and 500-series boards, but given that there's no indication whether the teased upcoming retail APUs will be these or some other solution (new silicon?) there's no indication if those will work on 400-series boards. Personally I wouldn't buy a 400-series board for these anyhow, for three reasons: iGPU/SoC VRM capacity (those new iGPUs is going to pull a lot more power than the older ones, especially when overclocked), RAM trace quality/speed support, and HDMI 2.1. All B550 ITX boards support HDMI 2.1, which makes them a shoo-in for a HTPC.
Just let me buy one of these, okay, AMD? Please?