A study in Finland had flaws in how it was set up and didn't account for certain medications, making it seem like swapping certain fats was better for heart health than it really was.

This study looked at the same diet change as the claim and found that when they only used well-designed studies without extra factors messing up the results, there was no heart benefit. This backs up the idea that earlier studies like the Finnish one had problems that made the benefits look bigger than they really were.