It made no sense to believe that games developed years before the PS5 even existed would be developed only for the PS5. It made no sense to believe that PS5 would not have cross-gen exclusives. All previous console generations have had cross-gen exclusives for the first couple years. These games are too expensive and take too long to develop, for it to be otherwise. It made no sense to believe that Sony would leave all that money on the table.
Sony fanboys made an unrealistic, global assumption (that no PS5 exclusives would appear on PS4) based on a vague statement by Ryan, combined with the journalist's headline. Ryan and the journalist share some of the blame. Ryan could've been clearer, and the journalist could have checked his assumption rather than just printing it as a headline.
But the Sony fanboys share some of the blame, too, because instead of exercising a little critical thinking, they grabbed this assumption and ran headlong with it into the console wars, waving the banner of "PS5 games will be PS5-only." Even though Ryan never said that.
So why the lapse in critical thinking? Well, the answer is easy. Because it felt good. Because it scored Sony fanboys points in the console war. Because it was what they wanted to believe.
So they got burned. That's what happens when we make unrealistic assumptions. It's silly to blame mean ol' lyin' Jim Ryan, because the Sony fanboys are just as culpable. They made a global assumption based on a vague statement, they spread it like wildfire rather than exercising a little criticical thinking.
I say this as somewhat of a Sony fanboy myself. Early on, I said several times that most PS5 games would be cross gen for a couple years. I was told that no, Sony has confirmed that they'll all be exclusive to PS5. That didn't make much sense (given the massive install base of PS4, the costs of these games, and the fact that many, like Horizon, had been in development long before the PS5 existed). But everyone seemed so sure, so I just nodded my head and went along with it. I didn't check the sources either.
Oh well. You know what they say about making assumptions.