When my X370 board died, bought an X470 so I could keep using my 1700x until I was ready to update the CPU, so an X570 was out. Gonna get burned on that one, as I was hoping to wait for the 8 core 4000 series to drop as the point to upgrade. :/
Possibly... AMD has learned to not over promise things in slides. I wouldn't write off x470 or eve x450 support until things launch. AMD is not going to announce blanket x400 support cause some boards may have ROMs that are just to small. I am sure though at launch the MOBO MFGs will announce a handful of 400 series that have support.
AMD not announcing every 450 board will work is expected. I am sure some boards will not be compatible, better to not announce then have confusion. Also I am sure the MOBO MFGs appreciate AMD not committing them to support work... and perhaps helping them sell some new 500 boards.