How to understand this description of 'collapse' in the Elasticsearch document？
First of all, thanks for providing a reproducible example, that helps a lot!!
Then, regarding collapse, indeed, it is only working on single valued fields. In your first document, title is an array, and hence, is multi-valued, which is not ok for collapsing.
Simply put, the 0 you see in the error message is the internal document ID, i.e. it's an incremental number that each document gets whenever it is indexed. In your case, 0 stands for the first document that has been indexed. If you invert the documents in your bulk call, you'll see 1 instead.