Ok, let me chime in an try to explain a few things about the VirtualBox Guest Additions (GA) problem first. I'll address the hyperthreading topic separately.
Modern Linux distributions (Fedora 33 included) install older versions of the VirtualBox kernel modules (e.g. 6.0.0) by default (as a fallback), but they are not used before any GA are installed. When the GA (from Oracle or from the Linux distribution) are installed, they build a new version of the kernel modules and install them. The Linux kernel 5.10 (used by Fedora 33) changed some kernel interfaces used by the GA, and therefore only the GA 6.1.18 (or newer) can be successfully built.
When you installed the GA 6.1.16, the kernel modules were not successfully built and therefore not installed (at least not
vboxguest.ko), but the remainder of the GA probably was installed, leading to a mixture of GA 6.1.16 and GA 6.0.0. Similar things happened to several other users in the past, but they usually didn't prevent the Linux guest from starting, just the GA wouldn't work as expected.
I don't know an intended method to prevent the GA from starting (and when I asked this a few years ago on these forums, there wasn't a specific answer). So we probably have to come up with our own strategy, and I'd suggest to do the following:
Remove (but not delete) the virtual hard disk from the Fedora 33 VM, and add it to another Linux VM as an additional disk drive. Boot the other Linux VM, mount the additional disk, move or rename all VirtualBox kernel modules (start with
find /lib/modules | grep vbox), optionally move or rename the other GA components (perhaps located somewhere under
/opt, use
grep -i vbox this time) and shut down the other Linux VM. Move the virtual hard disk back to the Fedora 33 VM. Good luck!
On Windows though it looks fine for now.