Oh yes, they are very different. Poach has two meanings: 1. is the way you cook an egg, and 2. is illegally hunting something not on your land, or something that is protected. Plunder means to steal from a place or person.

V-T If an organization poaches members or customers from another organization, they secretly or dishonestly persuade them to join them or become their customers. That's what my dictionary shows, as a third-place explanation of 'poach', while it doesn't show the similar explanation of 'plunder'. So they are not exchangable.