I agree with your sentiment, where NTS does have to be either a cuckold/cuckquean or a vixen/stag dynamic, where one partner derives pleasure from the other having sex with others. However, swinging literally just means consensual extramarital sex or partner sharing. Swinging is an umbrella term for all sharing of a romantic partner, spouse or otherwise, as long as it's consensual. Cuckolding, threesomes, swapping, orgies, gangbangs, etc., can all be considered swinging, as long as it involves romantic partners consensually indulging in said activities together. Even if one partner is only acting as a spectator. Therefore, NTS in it's purest form, is always swinging. It's when other elements such as NTR(Cheating) are introduced, that it can be considered something else.