I can say with 1000% certainty that myasthenia gravis can be caused by thymoma because I've read that thymectomy can be curative.
I've also read that Lambert-Eaton is associated with thymoma, but it's more closely associated with small (oat) cell carcinoma of the lung.
Although both LE and MG are associated with thymoma, in terms of greatest specificity, MG has the stronger relationship, whereas LE has the stronger relationship with oat cell carcinoma.
Another tid bit, which you may or may not have already seen, is that pure red-cell aplasia is caused by thymoma (Kaplan liked that). Be on the lookout for that cancer if the RBCs are down and the pt is otherwise unremarkable (one wtf-question removed).