That’s possible. I’m not an expert on navigation, but given that people didn’t have accurate maps, magnetic deviation might not have been as much of a problem. When leaving a port you only need to know what compass heading to follow to reach your destination, which you can learn by trial and error, and regardless of moderate magnetic deviations there will always be one such. It might not take you on an optimal route, but that’s less important than that it gets you there reliably.