The form action should be /addpost and not /posts (mistake in the article)

So your form in post_form.html should look like

<form action="/addpost" method="post">
        <label>Post Title</label>
        {{ postform.title }}
        <label>Post Text</label>
        {{ postform.post_text }}
        <input type="submit" name="submit">
</form>

Once you change that, you should see the post show up in the HTML as well as pgAdmin