AMD: Pre-X570 boards will not support PCIe Gen 4

Yes, that's what he said. Actually, AMD's Robert Hallock said that on Reddit. earlier on some motherboard manufacturers showed PCIe Gen 4.0 support in series 400 BIOSes. 

And that confirms it plain and simple. The new 500 series chipset will support PCIe express Gen 4.0 in combination with Ryzern 3000 processors. Anything below, simply not. Whether or not you'll find PCIe 4.0 a buying feature is also topic of debate, your GPU certainly will not need that bandwidth, and while PCIe Gen 4.0 based NVMe SSD(s) are fun, would you truly really need 5000 MB/sec in SSD performance, other than for bragging rights of course . 

"Hallock: This is an error we are correcting. Pre-X570 boards will not support PCIe Gen 4. There's no guarantee that older motherboards can reliably run the more stringent signaling requirements of Gen4, and we simply cannot have a mix of "yes, no, maybe" in the market for all the older motherboards. The potential for confusion is too high. When final BIOSes are released for 3rd Gen Ryzen (AGESA 1000+), Gen4 will not be an option anymore. We wish we could've enabled this backwards, but the risk is too great."