Even among Germanic/Northern Germanic languages you'd need some formal learning and practice to actually have some competency. Mutual intelligibility doesn't really cut it for "having learnt a language". I'd never say I speak danish or Swedish even though I can understand it somewhat.

The languages the man from the article has learned are considerably different from each other. Not all of them, of course. Still, even going from Spanish or French to Catalan and Portoguese is no picnic. Same for slavic languages. Flexibility with Grammar and already having the mental infrastructure can help, of course!