Is there an upstream source for this article? There's no mention of the actual versions of the Linux kernel, linux-firmware, microcode (either bios or applied by the kernel), etc. Which makes it seem less thoroughly investigated since I could easily see Ubuntu having something outdated.
I just got my 3900x a couple week ago and I don't have this bug according to the test code provided in the article.
I'm running:
> Archlinux linux-5.3.7
> Linux-firmware: 20191022.2b016af-1
> Microcode: v2.2 (patch_level=0x08701013)
> MB: ASUS PRIME x570-P. bios=1201