Why would the journalists assume that Dennison was Broidy? Didn't this story break after it was disclosed that Dennison was a name used by Trump in the Stormy matter?
As the story details, the existence on the NDA was leaked to a few news outlets. The WSJ ran it first and they had also been told the 'Dennison' in this case was Broidy. When they contacted Broidy, he copped to it right away. There was no denial, referral to a lawyer or a 'no comment' - just a straight up admission.
Did they 100% buy it? Who knows. However, they had someone on record, so they printed it. It wouldn't surprise me if they were digging further. After all, there is a somewhat recent precedent for this with Andrew Young taking the heat for the Rielle Hunter affair and pregnancy with John Edwards.