No the Cavoodle is not a purebred dog breed.
Actually the Cavoodle is a crossbreed which means that the Cavoodle is basically a hybrid of a cavalier king charles spaniel and a poodle.
The crossbreeding of the dogs to make the Cavoodle is done to avoid the genetic diseases that can be present in purebred dogs.
So when you get a Cavoodle and they say it's a purebred then they are not telling the truth or they don't know full well what breeds the Cavoodle is.
No Cavoodle can be a purebred because they are crossbred to make them.