I agree, the actor model is a significantly more usable metaphor for containers than functions. When you start thinking about supervisor trees, you start heading towards Kubernetes, which is interesting.
I think we're getting hung up on 'function' here: the article is really about the benefits of type systems (either runtime or compile time) as applied to the argument and result types of functions. The actor implementations I've used (mostly just Akka) lack strong types, but I think treating containerized apps as actors that can only send or receive specific message types would fix the problems the article brings up.