The argument that something IS secure is somehow supported by a whole lot of hypotheticals of the form "we could do $X and that would be secure if someone also did $Y".
So say it how it is: unikernels COULD be secure if all this work that needs to be done and we haven't done was done...maybe
That's par for the course. Unikernel people talk about VMs that use megabytes of RAM while cloud providers only provide gigabytes. Unikernel people talk about spawning VMs in tens of milliseconds while clouds take tens of seconds.
So say it how it is: unikernels COULD be secure if all this work that needs to be done and we haven't done was done...maybe