Once the decision has been made that Node 6 is no longer supported, it is then possible to refactor the code so that it uses appropriate modern idioms, such that the external behaviour is unchanged for Node 6+ but will no longer be parsable by Node 5.
In this specific case, it is both a refactor and a breaking change.
I think they use the angular definition which doesn't explicit forbid this
> refactor: A code change that neither fixes a bug nor adds a feature
But yeah I'm with you in saying that this particular example is not well choosen
> refactor!: drop support for Node 6
From Wikipedia's Code Refactoring:
> In computer programming and software design, code refactoring is the process of restructuring existing computer code—changing the factoring—without changing its external behavior.
So if a code change is just a refactor, then its external behavior is unchanged, therefore it is not a breaking change. These two commit labels are incompatible.