Which is why they are spreading the 300g out over an entire day, and it's the entire diet for 2 days.

The study is not suggesting this is a long-term diet. They're saying "eat oats for all your food for two days, and your cholesterol lowers by ~10% and then stays low for ~6 weeks due to changes in your gut biome".

They're not saying eat 300g for breakfast and then eat as normal. They're not saying do this every day.

They're saying 2 days, this is what you eat, spread out to replace all your meals across those 2 days, then go back to normal.

Yeah... a large (1" tall) canister of oatmeal is 1.2kg so imagine eating 2 big ass cans of oatmeal a week.

I think you are mixing up oats and oatmeal. And I think (but am not positive) that the study is referring to 300g of prepared oatmeal.

That wouldn't really make sense since amount of water could vary. Anyway the article says "Each oat meal comprised 100 × g of rolled oat flakes... boiled in water."