I think what they mean (and I agree) is that while dictionaries can help build vocabulary, and a better understanding of things such as handshapes etc.
Dictionaries can't teach language - they help in understanding components of language, but they can't teach the language. ... it's not just ASL, you can't learn English, or Latin, or Greek, or Spanish or any language from a dictionary.
The only real way to learn is to learn organically - to interact with ASLers, both native users and students, hearing, CODA, and Deaf
That being said - I have a ton of Dictionaries ... they're very helpful.