300g of oatmeal is about 3.3 cups (US measure).
I would consider a normal bowl of oatmeal for breakfast to be about half a cup, so this is quite a bit more.
Yeah... a large (1" tall) canister of oatmeal is 1.2kg so imagine eating 2 big ass cans of oatmeal a week.
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.