I guess I'm one of the few who views "slut" to be negative. And mind you, I do get the concept of reclaiming the word as a positive one in the context of poly, but for me it's a word that still have negative overtones. When someone calls me a slut, they're not doing so as a compliment.
So I guess I have to say that my take on the question of what is the difference between poly and slutdom in that sense. Just so you know my context.
Now for further clarification, I don't think sleeping with many people makes anyone a slut. Nor do I think that having sex with someone you don't love makes you a slut. I think that "slutdom" (in the negative sense of the word) comes from having sex in a random, inconsiderate, unethical, manner. It's more about the attitude of the person having sex than the sex itself, if that makes sense.
And in that context, I don't think having (passable but not great) friends-with-benefits-sex with a close friend makes you a slut. But I don't think it makes you poly either.
Poly is about building relationships - not just friendships with side benefits. IMO, anyway.
I think P is probably confused about what poly is, and I agree with someone else who said that from the outside (my) perspective, what you're doing is maybe swinging - or beginning to swing. But again. I wouldn't say it was poly at all.