The upper PCIe 5.0 x16/x8 slot on the ASUS ProArt Z790-Creator WiFi motherboard is positioned to allow the use of three PCIe slots, but with a slight limitation. If the x16 slot is used as x16, two additional PCIe slots are available. However, if the x16 slot is used as x8, three more PCIe slots can be used, but with reduced bandwidth for the other slots.

To summarise: There are effectively 3 slots available, but the configuration can affect the bandwidth of the other slots.

I haven't checked the sources regarding the post but the PCIe 5.0 x16 slot can be used as PCIe 5.0 x8, which is equivalent to a PCIe 4.0 x16 slot and thus enables the second white PCIe 5.0 x8 slot. However, you cannot use both PCIe 5.0 slots as x16 as they share the bandwidth and the second slot is deactivated in x16 mode.