Is the .then() considered the "resolver"?

I think you might be mixing up the semantics of "resolve". In the context of the way Andrew explains it, when a promise is "resolved/fulfilled", he's implying that the original function has yielded a proper value and not caused an error. This makes sense when you consider that then() can take an optional second argument, a function to be called if the promise fails in some way. We can also add the catch() method to the end of a chain of then() calls and catch() will display an error if any promise in the chain fails (which is helpful for staying DRY).

tl;dr: then() is the resolver when all goes well or it takes the optional second argument to handle all possible cases (the promise worked or not), otherwise it needs a call to catch() to exhaust all possible outcomes of the promise call.