"But, if that is the case, is the writer liable for libel in a court of law? That is my point. This guy would not have written it if he couldn't prove it."

That's what tabloids count on is people thinking that way - it makes them rich. Very few people in the public eye want to go into a court of law and sue. Because then you set yourself up to be crossexamined in open court. A lawyer can go where ever they want to looking for dirt. Better to just ignore it.