What's the trick to importing an entry with a categories field?
It's the modifyImportRow hook. If one uses that sample code in the Read Me, related categories will not import.

So we will have to run at least two imports, one with our Import Helper that uses the hook enabled  (for tables, booleans, dates, etc.), and one without, in order to append the related categories.

Unless someone can provide better example code to exclude an entry category field, but I can't think of how right now.