I heard hiv concentration is low at the starting of vagina (lips, clitoris...) higher inside cervix, is it true? is this one of the reason why cunnilingus is considered low risk by cdc?

It may be one reason: I don't know whether HIV concentration is low at the vaginal opening. I'm not aware of any studies of it, but it makes sense. However, that's probably only part of the reason that cunnilingus carries little or no risk of HIV. Saliva inactivates HIV, and swallowing infected blood or secretions is low risk. Whatever the reasons, there has never been a proved case of HIV transmission by cunnilingus.