Does it need le or la????

You are correct - you don’t use articles in French with ‘plus/moins X, plus/moins Y’ expressions. The lesson is basically explaining this is not done. One of the situations where for a change French doesn’t need an article but English does - and that is why the English translation includes the article as in standard English speech. That’s grammar, no matter what language you are learning  - grammar often has no particular reason, other than ‘it just is’.