Basic orbital mechanics. You cannot raise the perigee of an orbiting body by giving it additional velocity at perigee; you can only change its apogee or inclination. Similarly, for a body in a circular orbit, you can either lower the perigee (leaving the apogee at the original altitude), or raise the apogee (leaving the perigee at the original altitude). The relevance is that it means that the debris will still spend part of its orbit at low altitude and will thus decay relatively rapidly.

NASA did not 'get it wrong'. They said (as I did) that it was an irresponsible act. I don't think that they have predicted that the debris will be highly persistent in the way that certain previous anti-satellite missile have been. They did, however, point out that solar minimum was a particularly bad time to have done this, which I hadn't thought of.

Thank you. Am I correct in assuming that some debris will pass through the ISS orbit altitude?

Do you assume constant energy? If so wouldn't the interceptor have increased the overall energy of the system?

I couldn't say, but all the people who are in a place to track the debris seem to think that at least some of the debris has an apogee higher than the ISS. I would imagine that the risk of a collision is fairly small, but it still seems unnecessary to have raised the risk at all.

No, you don't have to assume a constant energy. Any input of energy at perigee cannot influence the altitude of the perigee regardless of how much you add. Even so, assuming that the warhead was kinetic and the impact was close to head on (as the animations suggest), then the mean orbital velocity of the constituent parts of the satellite should have been lowered slightly. Nevertheless, the distribution of energy between individual fragments would have been very non-uniform, with some particles re-entering, and some being given a lot of additional energy.