I was taught UML activity diagrams were high-level representations, while this is a (summarized) flow chart of code paths (which makes it quite big in comparison to what would be its high-level UML counterpart).

UML is supposed to be an architectural diagramming standard. What's the point of diagramming in UML the actual code paths?

E.g.: you'd never have object shapes or function calls in an UML activity diagram.

This is just an ad-hoc flow chart AFAICT.