As one in an open marriage, I find it somewhat a bullshit statement for swinging. While some swingers may play alone, I feel playing together is the default, not the exception, in swing-land.
I think there's an entirely different mentality involved with being able to go off and play alone versus swinging together. Some swingers may be able to do both successfully, but personally, I'm not very fond of that statement as I am sure there are plenty on this board who might find it difficult to let their SO go off on their own (be it another room or completely separate), but who really thrive in a traditional swing environment. Obviously others' mileage may vary.
If the statement was "If you can't handle seeing your partner enjoying sex with another, then swinging isn't for you", I'd be more inclined to agree.
Just my .01.