Embarrassingly, I didn’t know about that plugin, so I added this to the template head layout:
<meta name="twitter:site" content="@{{ site.twitter_username }}">
<meta name="twitter:creator" content="@{{ page.author }}">
<meta name="twitter:title" content="{{ page.title }}">
{% if page.summary %}
<meta name="twitter:description" content="{{ page.summary }}">
{% else %}
<meta name="twitter:description" content="{{ site.description }}">
{% endif %}
{% if page.image %}
<meta name="twitter:card" content="summary_large_image">
<meta name="twitter:image" content="{{ site.url }}{{ page.image }}">
{% else %}
{% endif %}
Since it uses summary, it works with the usual nbdev summary. Although if description is more common, it’s possible nbdev should set that metadata too…