Hey, both answers are correct... You can dive the Maldives all year, especially on a liveaboard as they can move around depending on the season. Yes, May - Nov there is more plankton which = manta and whale sharks. Best time for that is August I believe, so both answers are right :oD
The beauty of the Maldives is that you have literally thousands of options of dive sites, spread over 100’s of miles. I always travel to the Maldives in August for a Whale Shark study (this year included so very excited as now counting down) and we usually use the MV Carpe Diem boat which I can highly recommend. They schedule their trips based on the season and have very good dive guides as well...
Good luck and if you have options, a liveaboard is best as you can see so much more that staying in a hotel :oD