I think part of what the article conveys and I think lacking in many of the messages that just say "don't roll your own crypto" is that testing and proving thin

I think part of what the article conveys and I think lacking in many of the messages that just say "don't roll your own crypto" is that testing and proving things is nowhere near sufficient to write a secure crypto library. The hard part is all the invisible things like the side channels. These are the pitfalls that you will fall into because you must know about them to avoid them.