True. I generalized.
Now if you are a swimmer with poor stroke efficiency as most triathletes are, than it will be more of an aerobic workout.
Ahem. Most triathletes, at least the ones I hung out with, are maniacal about stroke efficiency - even more so than the swimmers I know. It is the same way on the bike - we are way more paranoid about aerodynamics than just straight bikers. We know that we have to maintain muscle energy and fuel for the long run - so we'd rather be efficient but maybe a bit slower, rather than flail around with better speed.
It's why we kick slower in our swim style, too. This may be why I had drilled into me that swimming is anaerobic - because for a good triathlete, that's the case.
Please check out Total Immersion Swimming.
Yes, that is a very popular system with many triathletes I know.