I think you meant polyamory where I highlighted ‘it’s swinging’, but I know what you mean.
I know the definition for sure. I have been in both types of relationships. And you hit the nail on the head with ‘play’. Never have I ever heard true poly people refer to their partners as play partners. But it’s highly prevailing in swinging.
And again, nothing at all wrong with either lifestyle. Just don’t combine the two in your (collective) mind and call one the other, just to make yourself feel better about downing them.