Heh reading the article it is not clear to me what the difference between abstraction and formalism is. If I have to guess from the colloquial usage of the word

Heh reading the article it is not clear to me what the difference between abstraction and formalism is. If I have to guess from the colloquial usage of the word, abstraction is more about removing details, while formalism is about adding elements/details which do not originate from the domain. Maybe people often arrive at the same abstractions but formalisms are often unique. In your example clustering is probably an abstraction but the rules for choosing names might be a formalism. Unsupervised vs supervised learning is another comparison that this connotes..