Strongbox technically is very strong, without a doubt. But, being TOR based, it will be hard to use. Worse, a potential leaker not only must use their own computer (ideally a throwaway computer), but they can never have VISITED the Strongbox information page from work, because otherwise any leak to the New Yorker will be suspicious.
And Strongbox's information page drives Ghostery crazy! Not a good sign for a privacy tool.
Probably more important is general Operational Security, including burner phones and/or burner computers.
Julia Angwin has an excellent additional point: Physical mail (dropped in a random post-box with a bogus return address) is perhaps the best way for anonymous one-way communication. The USPS will record address information when asked by law enforcement, but (currently) doesn't record this on all mail. Thus there is no history and, even if there was, this can only be traced to the processing post office. Perhaps the best use of the mail is simply to send the reporter a burner phone preprogrammed so that the reporter can call your burner.