It's still clickbaity, but the title implies a comparison between a very lightweight VM and a heavy-weight container (presumably a container based on a full Linux distro). You could imagine an analogous article about a tiny house titled "my house is smaller than your apartment".
I could see it being an improvement over other VM control planes, but docker still wins in performance for any equivalant comparisons.
A unikernel can definitely be compared.
Linux containers are essentially the separation of Linux processes via various namespaces e.g. mount, cgroup, process, network etc. Because this separation is done by Linux internally there are not that many overheads.
VMs provide a different kind of separation one that is arguably more secure because it is backed up hardware -- each VM thinks it has the whole hardware to itself. When you switch between the VM and the host there is quite a heavyweight context switch (VMEXIT/VMENTER in Intel parlance). It can take a long time compared to just the usual context switch from one Linux container (process) to another host (process) or another Linux container (process).
But coming back to your point, no firecracker VMs are not lighter/lightweight than a Linux container. They are quite heavyweight actually. But the firecracker VMM is probably the most nimble of all VMMs.
No one cares how the language runtime is actually running.