Yes, that's right. They can't be friends. Unless both of them are involved with other people. Then they can. This is an amendment to the earlier rule.
10 seconds sound clip from the When Harry Met Sally... movie soundboard.
You can hear this line at 00:21:32 in the Blu-ray version of the movie.
Quote context
[...]
- Would you like to have dinner? Just friends.
- I thought you didn't believe men and women could be friends.
- No, no, no, no, I never said that.
- If two people are in relationships, the pressure of possible involvement is lifted.
- Like it means something is missing from the relationship and you have to go outside to get it.
- I mean, come on, who the hell are we kidding? Let's face it.
[...]