What's not reasonable about building from source? It really seems like a small cost to snapshot a version of third-party code into your own source control system. There really aren't (or shouldn't be) that many different things in your environment.
The problem here is economic. Because there is no body of information about software assets acting as public good, each individual is forced to internalise the costs. Either you internalise the cost of building everything yourself, or you internalise the risk of accepting incoming assets on faith. But you bear a cost.
However these modern stacks contain a lot of small parts. The article gives the kiwigrid/k8s-sidecar as an example. Also, well shown in the article, it is not as easy as copying some commands to your own Dockerfile. Look at the busybox image using glibc from another debian image.
It's not reasonable in the sense that it is a hell lot of work and would require more time and effort the average devops/sre/whatever (team) has.
It would also go against the promise that Kubernetes would make things easy because you could just do helm install stable/prometheus