Title text:
They initially thought the containment structure alone would be sufficient, but for good measure they decided to include the Elephant's Foot inside.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3316/
What does the elephant's foot have to do with this siege approach
The elephants foot is the name for the melted core at Chernobyl, its relevance is that the siege equipment in question is the Chernobyl containment facility (which was built to the side and then rolled over the entire reactor building because you can’t build in place with all the radiation trying to kill you).
So, containment facility is an interesting siege weapon, including the elephants foot turns it into a nuclear attrocity in a big box, rather different vibe.
Xkcd always has another punchline in the alt text, in this one they say they'll include the elephant's foot.