How is this a noun?

The website is poorly written. A gerund is not a "noun": it acts like a noun in most but not all contexts.

Like a noun, a gerund can be the subject or object of a verb but a gerund describes 1. "the action of the verb", and 2. it is qualified by an adverb.

1 "Swimming keeps you healthy." = the action of the verb to swim keeps you healthy.

2 "Swimming quickly gets you healthy." - Note that "quickly" modifies "swimming", not "gets."

Compare with the noun "exercise":

1 "Exercise gets you healthy."

2 "Exercise quickly gets you healthy." - Note that "quickly" modifies "gets", not "exercise."

Edited to change "keeps" to "gets"