If you instantiate it with concrete types, does "(b IntBox) Map(f func(int) string) StringBox" make more sense? You have a collection (in this case Box) containing values of type T, a function that maps values of type T to type U, and if you apply that function to all elements in that collection you get a collection of type U.
Only they are not using a collection and therefore "Map" was a confusing choice for the method name (maybe "To" would have been a better name?).
A one item collection is admittedly a lame one, but it is a collection. Pointers are collections of zero or one items in a sense.
Now, the verb "map" is the traditional name for this operator, but I agree it sounds confusing when the noun "map" is also a collection type. C# and SQL call this Select, if that helps.
Is that really the confusing part? If you had a general collection interface, say Mapper, a Box (i.e. a singleton set) could implement it just as well as a List or a Tree.
I realize that mathematically speaking a singleton set is still a set, but as a programmer who is used to map operations in other mainstream languages, I was expecting some kind of loop in the implementation. It took a few page faults in my brain to realize what's going on.
Your understanding is quite incomplete. Wait till you think about the mapping operation for functions. Then there’s no loop, just function composition.