You can't truncate full_text as it's enriched with django CMS frontend editor markup (when in editin mode)
You can change this to something like:

    <div class="blog-lead">
        {% if not TRUNCWORDS_COUNT %}
            {% render_model post "abstract" %}
        {% else %}
            {% render_model post "abstract" "" "" 'truncatewords_html:TRUNCWORDS_COUNT'  %}
        {% endif %}
    </div>