There are benefits and drawbacks to either approach. Some simply can maintain a better narrative flow if they wait until they are finished to add the sourcing. They may have a stack of sources sitting beside their computer, but not be sure which will be relevant until they are well advanced intheir writing. Presumably they are making private notes as they go along so that they will be able to add the references when they get to the bottom of the article. If an article appears to be a work that is actively in progress it is good etiquette to let the editor finish what he is doing before complaining about missing sources.
Oh certainly, you don't need to add the sources to the code as you go along, but you need to know what the sources are. The sources should be added in the same editing session as the article is written, the exact order within that session is up to the writer. Someone else coming along after you've written the article and trying to find sources for your statements is completely wrong - it's not what "source" means. They are effectively rewriting the article.
If you want different people to do the sourcing and the writing, then the sourcer should find the information and add it as bullet points to the article, with correct references, and then the writer comes along and makes it read eloquently. The other way round is completely wrong.