No that's not true. It depends on the individuals interpretation of the 2 differing phrases.

For example, if you said...."I could not care"... then this means that you absolutely without a doubt 100% do not care about something.

For some people this is the most efficient and relevant way of communicating to someone else that they do not care. So why would you add the word "less" at the end of the sentence "I could not care" making it "I could not care less"? For some people this doesn't make sense and to others it does. To some people "care" and "care less" are total opposites in their idiomatic ways of rhetoric.

To some people, saying "I could not care less" literally means "I could not not care". It means this to some people because some people look at the words care and less when combined as "care less" as a whole phrase even though they are two separate words for obvious reasons. To some people "care less" means the same as "no care" because it is an idiom. Hopefully I explained it where it makes sense and if it doesn't that probably means that some people prefer a different color than others or don't see things in the same light. It's a very subjective and expressive subject that we are talking about here.