I don't care how people pronounce it. I just insist that it's a non-count noun.

Too many people say things like "these data" when they'd never actually say "this datum". Most people, if they ever talk about individual items from a dataset will say "this piece of data". That's how you talk about an uncountable item: this grain of sand, this drop of water, this slice of bread, this cup of tea, etc.