I think it's meant to be parsed as "I liked being able to (express (calling external libraries))", not "I liked being able to (express calling) (external libraries)"

Yes. Probably could’ve written it better but I’ve not poked around F* in a while and I was writing on my phone. Say you’re calling an external function implementation in hardware, being able to express those interfaces as external makes it viable to use F* vs assuming everything is open source and introspectable.