> These scientists could not believe the two Curies more.
It's easy to cause confusion. In English, we often say "I can't agree more", meaning "I wholeheartedly agree with you". But "I can't believe them more"? Mmmh, I think this author intends to say "absolutely believe them". But in real life, who would say that weird sentence? If you read it too fast, you would even think it was "... any more", which is the opposite.
It appears that, not all verbs can be used in the structure "somebody can't [verb] more..." to mean "[s]he very much [verb]'s...".