There is a distinction.
To say it was shown "conclusively" that the attack was a fake, is wrong.
The issue is that no one has been able to conclusively show that the Assad regime caused the deaths.
Big difference.
A few minutes on the internet can elicit intelligence agencies and weapons experts who agree that, although it is possible Assad was behind the chemical attack, the evidence is sketchy.
Pulitzer Prize winning reporter Syemour Hersh, on the German news site Die Welt, carries the story further, reporting that Trump was “warned by the U.S. intelligence community that it had found no evidence that the Syrians had used a chemical weapon.” 
Basically, the impetuous an uninquisitive U.S. President  pulled the trigger without all the facts.