People say that because it contains ~25% more ZDDP than API oils. The reason the phosphorus (from ZDDP) was restricted to 600-800 ppm was due to concerns over phosphorus and sulfur causing damage to emissions equipment such as the catalytic converter.
The reality of it is that it's a massively dramatized issue. Yes, phosphorus and sulfur can damage catalytic converters if large amounts of it are passed through the cats. I'm talking about a quart every 500 miles kind of consumption through the engine into the cats. So now the cat will last only 400k miles instead of a potential 600k miles. Doesn't really matter when 99% of vehicles end up in a junkyard with
I prefer what's good for the engine, not what's good for the cats. If they are at odds with one another, I'm choosing engine life over cat life.