I'm GUESSING (I haven't asked him directly, although I could) the reason that Manning has changed his position is:
1. He now has a base of radical Black comrades with which to dialogue and work with, which is better for his mental health. The stuff you describe is pre-BRC, and post-Rainbow Coalition. Manning was suffering at the time from both the betrayals and the lost promise of the Rainbow Coalitioon, and the lack of a strong base of fellow Black Radicals to give him a true sense of hope. From that base, it's easier for one to branch out and try new things. No, I don't know this for a fact, but as a Black man, I understand a few things about how the Black mind works.
2. In spite of the shortcomings of people like Ralph Nader and the Greens, Manning probably trusts people like Nader and other Greens as basically decent human beings in a way that he didn't the aforementioned CoC colleagues. Once again, this is just speculation (I haven't asked Manning about this directly). I'd be more than willing to bet that Manning doesn't miss any of the folks who left the CoC back then.
3. In 1992, the DLC had not as yet completely taken over the party in such an overt manner, so there was still some hope that the party was worth saving. Also, in 1992, a lot of folks were in denial or unaware of just how much of a "centrist" Clinton would turn out to be.
4. We had dealt with 12 years of Republicans, and it was not the time to risk going for 16 years. Now that we've had 8 years of Democrats acting like Republicans, the risk is worth it. We no longer have as much to lose, as evidenced by Clinton's record.
Basically, the objective conditions have changed, and Manning has changed his position along with them. Unless I missed something in Marxism 101, that's what he's supposed to do.
Art