HoudahSpot searches the Spotlight index. This relies on importer plug-ins to extract content and metadata from proprietary file formats. Such importers are usually included with the application that owns the file format.
I don't know the Drafts application. I don't know if it includes a Spotlight importer. I don't know if the Drafts application creates individual files or works as a "shoebox" where all your documents are in a single file. Spotlight only stands a chance if there are individual files and a Spotlight importer to read these.
Some of the "shoebox" application use a different technology - confusingly called CoreSpotlight - to get results to show in the Spotlight menu. These are not actual files, but entries in the shoebox database. You can tell the difference when you scroll down to the bottom of the results in Spotlight and select "Show all in Finder". The results that are missing once you get to Finder are those that are not actually files.
Since HoudahSpot is a file search tool, these results are off limits. HoudahSpot expects to be able to show file paths, sizes, previews, modification dates, etc. and let you copy, tag, and rename files. All of which is not possible if you are looking are results that are not actually files.