Duolingo is for getting to an A2 level. It’s hard for most people to get much value from trying to read a book or talk with someone unless they’re at least at A2 level
That's bad news for the millions that had to do just that and learned fine in fact better. I'm not saying you should read the Upanishads in the original, I'm saying a proper intro textbook is fine and some teacher-figure ("human") to actually use your body and read theirs.
Heck, Latin even has a Latin-only method: LLPSI. "Roma in Italia est", you'll figure it out quickly enough.
As someone who had to learn languages from textbooks back when it was the only thing that existed, it was massively ineffective way of learning.
The typical outcome was a person who spent a lot of effort and still was unable to do anything useful with the language after years (reading real book, watching movie, chatting).
Notice I included interaction with people, but you're right. I think just using books is about equal to just using apps. Both won't get you there but books will get you further along the way.
Heck, Latin even has a Latin-only method: LLPSI. "Roma in Italia est", you'll figure it out quickly enough.