Block storage (virtio-block) is way simpler than file systems, both in the virtio implementation and in the host filesystem implementation. Simple is good, both for security reasons and because it makes it easier to reason about performance isolation between guests. The cost is obviously in features, and in making it hard to share data (and very hard to multi-master data) between multiple VMs. To see how simple virtio-block is, take a look at Firecracker's implementation (https://github.com/firecracker-microvm/firecracker/blob/mast...).
The existing alternative, as the post talks about, is 9p which has a lot of shortcomings, so this is interesting. I suspect it's use is mostly going to be in development environments and client-side uses of containers, and server-side users of virtualization will likely stick with block (or filesystem implemented in guest userland).