Given the relative large sample size of operas under Bonyge that I listened to (BTW, I missed La Traviata from the list), that statement is the most likely to be accurate (i.e., the point estimate using the maximum likelihood method). Perhaps you can provide some weight of evidence by going through all the CDs you have with Bonyge conducting, which I am sure would be a very pleasurable experience for you, and give us a counter-example. :)