You can't group all species of dinoflagellates together, and you also can't compare what happens in our small, closed systems to the ocean. Yes, ride tide is caused by excess nutrients, but it is only one species of dinoflagellate, and K. brevis is not one of the three common pest dinoflagellate species in our aquariums.
There is a huge abundance of anecdotal evidence that suggests that dinoflagellates appear in reef aquariums with little to no nutrients. They are one of the big plagues of ultra low nutrient tanks, for starters. How can a species that ONLY thrives in high nutient environments like you suggest survive in tanks with intentional ultra low nutrient input?
The theory is that the problematic dinoflagellates in our home aquariums can't compete with the microbes in a healthy tank until there is too little nutrients, in which case the normal microbes get weak and die while the dinos take over.