The core underlying issue is that Japanese doesn’t actually have present/future/past tenses, at all. It has completed and uncompleted, and a specific construction to cover (amongst other things) ongoing action.
In this instance, your knowledge is ongoing, hence the -て form, but completed, in the sense that you already “are knowing”, as opposed to “will be knowing”.
Ohhh, now i get it so this is what they mean when they say thay certain verbs used with ている describe the result of a change, something that retains its significance until the present. You just made it way more clear for me, the book’s explanation was a little confusing. Thanks a lot!!