apps.articles.models contains Articles and Article page types, where the naming has come from initial development. In user-facing pages, however, and in the Wagtail Admin, we are preferring to use the term "Posts", to give us more semantic flexibility with what we can do with these pages.
This re-labelling may or may not be permanent.
As a result, the changes done were all kept (as part of ticket 624) at surface level, with no table-altering migrations and no change to the CSS classnames which mention article. Similarly, no template names were changed, so you'll find articles.html and article.html not post.html, etc.
Streamfield blocks have been renamed from 'article' to 'post' to keep the UI consistent, but the internal use of a type or resource_type attribute on a Page instance has been left with the value 'article', not changed to 'post' because that would have been a non-visible-to-humans change.
However, in the future, we may decide that we definitely want to stick with "Posts", in which case a proper model-renaming exercise is worth it, along with CSS and template-name changes, too.
While Wagtail Search will auto-update the index when a page is saved, and while there is also a scheduled task that runs once a week to rebuild it all, if you're setting up a new build or have just loaded data from a DB backup or the like, you will need to manually rebuild the search index:
yourhost $ docker-compose app sh
/app $ python manage.py wagtail_update_index