On other website, there's info, that i9-9900K will be most probably supported only by Z390 boards.

I would think they would want to advertise the i9-9900k being compatible with the Z370 boards. Wait and see if that is the case I guess.

First, these BIOSes are not new, they have been made available weeks ago. Second, the alleged CPUID for the alleged 9900K is listed inside the BIOS, so yes, they seem to be compatible Third, these BIOSes have the microcode/hardware support for (Spectre) Speculative Store Execution, but you need to manually enable it in Windows if you want it active OS-side Fourth, these BIOSes upgrade the Intel RST EFI module to 16.5 from 16.0, so you should also probably use the corresponding 16.5 drivers. I'm using one of them on my board for a while now, all is fine. I had to raise the voltage on VCCIO with 0.01V (1.15 to 1.16) though as for some reason, XMP was no longer stable with former values, would fail Blend and IBT. Now all is back to stable.

My MSI Z370 Gaming Pro Carbon is already flashed with the latest BIOS (E7B45IMS.A60). There are also two new MSI motherboards in the leaked support list, the Z370-OC Pro and the Z370 OC Gaming.