harry potter - Which Side Did the Slytherins Ultimately Fight For at the Battle of Hogwarts?

The Slytherins came back and fought against Voldemort in the final battle, with Horace Slughorn leading the charge!

JN: And how much is it that being sorted into Slytherin is, you know, sorted into good guys and bad guys here?

JKR: Well, they’re not all bad, that would- I know I’ve said this before, (JN: Yeah, I remember.) and I think I said it to Emerson [Spartz - Mugglenet.com], they are not all bad, and, well, far from it. As we know, at the end, they may have (laughs) a slightly more highly developed sense of self-preservation then other people because…

SU: Yeah, right.

JN: Yeah.

JKR: A part of the final battle that made me smile was Slughorn galloping back with Slytherins, (SU: Yes!) (JN: laughs) but they’d gone off to get reinforcements first, you know what I’m saying? But yes, they came back, they came back to fight, so I mean- but I’m sure that many people would say “Well, that’s common sense, isn’t it? Isn’t that smart, to get out, get more people and come back with them?”

Pottercast # 131 -- The Leaky Cauldron -- 01.02.08

In this context, JKR's explanation leads me to conclude that Phineas Nigellus was referring to Slytherin House as a whole when he made his comment about "Slytherin having played its part."