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You can do that where for instance the VM has PCI and can plug in a virtio RNG device. But a lot of the QEMU setups Guenter Roeck reported issues with are emulating real (usually old) hardware which didn't have a PCI bus, or any other hardware RNG interface.

Also lighter systems such as routers (E.G. many OpenWRT targets)

https://wiki.qemu.org/Features/VirtIORNG

Yes, and that is commonly used.

But Linux must continue to work even on VMs without that, and that is the hard part.

The VMs described in the article are (faithfully) emulating real hardware lacking unpredictable timers or other sources of entropy. It’s not exclusively a VM problem; it’s a weird/old hardware problem, too.