I think a lot of that is baloney. Not all apps require the same degree of rigor. In fact most require quite little. If a library is widely used, that's a pretty good indication it's "safe".
A functional prototype is software that implements its functional requirements (e.g: features) but not its non-functional requirements (maintainability, stability, scalability, performance, configuration, monitoring, security, etc.)
What is "a lot of baloney" is people selling functional prototypes for the price of a finished application, or not knowing the difference.