Anonymous sources that are quoted directly are known to the journalists; their identity is only concealed from readers. This is about anonymously passing information for further investigation.
You may be thinking of "unnamed sources" or "anonymous sources". Those people may be quoted on a factual basis, but the reporter has an obligation to obtain the same kind of confirmation of their information as any other investigative lead.
Actually, that's not a tip by the NY Times definition:
A strong news tip will have several components. Documentation or evidence is essential. Speculating or having a hunch does not rise to the level of a tip.