He says that to denigrate the press so that no matter what TRUTHS they report about him his devoted worshippers are programmed to not believe it. That is his only goal. His only motive. He consistently undermines the credibility/worth of his "enemies" so they will be reduced to liars and he will be absolved of anything he is accused of because he will have brainwashed the people that it be so. Isn't he a swell fella? Everybody says so. Everybody. Just ask him.
Once upon a time any journalist worth his salt would throughly investigate any stories brought to him. Nowadays the press merely regurgitates what the establishment feeds to them.
Good for the prez calling them out.