Communication (Mind-Reading)

If two people have been in a relationship for an extended period and are unable to know what the thoughts and or behaviors of one or the other are in every circumstance or situation without it having to be “spelled out,” does that mean that they don’t know the person at all? Perhaps, that is one of the messages implied by the 1970’s hit song, “If You Don’t Know Me By Now…” It infers that if two people are in a love relationship over an extended period they should know each other quite well; but if they don’t know, given the lengthy time in the relationship, then perhaps they would never know each other. Is the song correct?