I think we are needlessly complicating this. We are talking about the
use of a term in an internal code base. While I would agree heading is
more correct, I don't think it is such a big issue to use headline if
that make the transition to a consistent usage easier. When it comes to
code, I think consistency trumps correctness.

I agree. I think the original post was about inconsistency in the documentation. We should just correct the documentation, use the word heading consistently throughout, and mention at a prominent place that the headline symbol is used in the code to parse headings. There is nothing wrong with that.