FDA has approved Epidiolex, which contains a purified form of the drug substance CBD for the treatment of seizures associated with Lennox-Gastaut syndrome or Dravet syndrome in patients 2 years of age and older. That means FDA has concluded that this particular drug product is safe and effective for its intended use.
I will take the FDA's word that Epidiolex is made from CBD isolate, and not full or broad spectrum. They have all the data in the world on Epidiolex, so I think they would know.
Here are articles also mentioning that Epidiolex is made from isolate CBD