I was also surprised that the information was provided in response to my request, but I believe boat brokers do have access to global information on boat sales (analogous to a the real estate multiple listing services in the US?). It is possible, however, that the information could be specific to that particular broker, from their own records. Given the number of transactions, and the number of brokers in the same competitive space, I doubt that though.
In any case, I did not have direct access to the source of the data, and cannot vouch for its accuracy or authenticity. As far as list prices go though, they are not out of line with current listings, and the differential between list and sale price does not seem to me to be illogical or remarkable.