Not reallly true. On a derlivery trip, I navigated from 125 miles off the coast of the Yucatan, right into the jetties at Progresso, Yucatan with an Iphone 4. The boatowner did not have the charts for Yucatan in his big chartplotter- he "wasn't GOING there", so all it showed was a blue line for the coast. He hadn't planned to rip a sail up either.
Then the Iphone got us around the corner to Isla Mujeres, where he DID have the chips
I wasn't aware one needed a cell signal to use a standalone gps... but in any event if you have a magnetic compass, a timepiece and a paper chart you can get anywhere.