Trump does have a point here. The standard for libel for public figures is “actual malice.” That means that, for example, an outlet like “Buzzfeed” can get something like the “Piss dossier” from a dubious source and publish it, without any attempts at verification. Just a shrug of the shoulders and “we thought it was newsworthy that this is out there.” Once a marginal outlet like “Buzzfeed” runs it, more legitimate news organizations like CNN can run it. Then the so-called mainstream media are free to run the original allegations and weeks of stories about the reactions to the original allegations.
Lest you think that this is fine, realize that it works both ways. Defamatory allegations can be made about liberals and Democrats, too. And under the malice standard, the victims have no recourse. Consider Pizzagate. The owner of the pizza shop, not a public figure, could conceivably sue someone for slander/libel. But the various public figures named have no real recourse because they would have to prove actual malice against any outlet they sued.
A reasonable standard would be that a news outlet that ran a libelous allegation would have to prove that it made good faith efforts to verify the allegation. And “two to four former and current US officials” wouldn’t cut it. If the sources are unwilling to publicly stand behind scurrilous claims, the media shouldn’t run it.