Anaerobic vs Aerobic training should not be considered exclusive of one another. They are complementary pieces meant to maximize swimming efficiency over a given race. Specificity of training for the purpose of racing is the efficiency of timely, useful work toward achieving your performance goals.
As a high school swimmer, coaches would constantly refer to the need to build an 'aerobic base' for future swimming success in college. As a coach, I heard the phrase shift from 'base' to 'capacity'. The clear sustained implication that copious amounts of 'work' were a pre-requisite to reap long-term benefits that may or may not be correlated. Only now do I understand that the aerobic capacity'is a function both of cardiorespiratory performance and the maximum ability to remove and utilize oxygen from circulating blood.'
Clearly, the solution to performance plateaus must be sports science. So we did the submaximal work. The same ridiculous collection of CO2 and VO2 gases to determine a specific VO2max from graded, stepped efforts of long breaststroke swims – now what to do with it? The man was ‘fit’. After all he was an Olympic athlete.