Morpheus was sjown to be wrong on many points, this may be another rationalisation he made. He may have assumed or accepted that it was the reason for being kept in the simulation, without really knowing the truth.

It could be that the characters in the movie thought it was about energy, but were mistaken. (But to be honest, having a group of people believe that to be the reason is just as implausible as it actually being the reason - either way it makes no sense and we just have to suspend disbelief.)

It was only Morpheus that said that. It could be that Morpheus was taught wrong. Any basic understanding of science would show that Morpheus’s statement was wrong. But his knowledge of science was learned inside the Matrix.

The movie’s were all about revealing layers of truth so it would have fit right in with the theme.