I’m not a huge fan of changing the impl. To me, there is a natural ordering, you determine the block boundaries, at which point you know where the link reference definitions are, then you can do inline parsing. The proposed change would mix and interleave the stages. As I understand it, you’d have a first pass where you detect block boundaries with sufficient precision to identify link reference definitions but not distinguish list items from headers, then do the link reference definitions, then a second pass where list items and headers can be distinguished.
The example given is an edge case that is very unlikely to occur in real documents. I don’t think it’s worth complicating the implementation or the conceptual model over parsing, unless there’s a case that it really is more natural for humans, or where there is a compatibility concern.