Is the plan in game wrong? I could not find a switch/upgrade path from the Epheria Sailboat to the Epheria Frigate.

The player has to choose from the Bartali Sailboat what upgrade path to go. But when someone already has the sailboat, there is no switch to the frigate.

So the correct path has more likely to look like that?

correct, you choose either the epheria sailboat or the frigate, can't change between them.

The graph is indicating that you can turn a Bartali Sailboat into an Epheria Sailboat OR an Epheria Frigate, not that you can swap between them.