In that link you posted there are some explanation of using Instrumental, this is the case here: [Instrumental is used ] with certain verbs "być" (to be), "zostać" (to become), "interesować się czymś/kimś" (to be interested in something): Basia jest dziewczyną - Basia is a girl. Adam jest chłopakiem - Adam is a boy. Piotr jest przystojnym chłopakiem. - Piotr is a handsome guy.
Just to remember - why Instrumental not Nominative, I would say it's like this "to be" would be actually mean "to be in a role of something".
Btw. I'm native Polish speaker, if you would have any questions - I will be happy to help. :)