I had a look at the github repo. The summarization is being done by a library called Sumy.

After a quick look through it, you're right that it doesn't use machine learning. However, it does use a lot of key concepts from Natural Language Processing, such as Tokenization, which is a subfield of Artificial Intelligence.

You're right, I had a look too and couldn't find much. The Sumy space is hosted on hugging face though, which does ML stuff. But yea can't be sure