Yes, because kræfter means strength. My friend Jens used to use this word when he wrote his emails to me, Så,nu rækker kræfterne ikke længere. Mine fingrene vil ikke mere. He had MS and when he got tired he would sign off with this word, meaning he no longer had the strength in his fingers to do anymore.
So I just finished talking to my good friend Ingerlise, who is a native Dane, and she has said given the English sentence, "The boy has not got more energy, she would have used energi. I understand that you can have energy but not strength, but in the case of this sentence, there is not way to distinguish. I feel energi should be accepted as well.
In case you, or someone else, still need this...
'Flere' is used for countable nouns and 'mere' is used for uncountable nouns, such as: liquids (milk, water); abstract ideas (advice, chaos, motivation); powder and grain (rice, wheat, sand); mass nouns (furniture, hair, transportation); natural phenomena (sunshine, snow, rain, weather); states of being (sleep, stress, childhood).
So, that's why someone asked whether 'energy' is countable in Danish. I think that in this use of the word, it should not be. If it were referring to kilowatts, then it would be. IMHO. But, I also agree that 'energi' is the word that should be used here.