It is a common saying, but the saying is "couldn't care less"

They are commonly used synonymously. English as she is spoke is not a logical language.

https://www.merriam-webster.com/grammar/could-couldnt-care-l...

https://dictionary.cambridge.org/dictionary/english/could-ca...

Try:

"It is impossible that he could care less." — The Morning Post (London, Eng.), 18 Jul. 1840