It’s about velocity, not altitude. The pieces from the explosion would need to get a fairly dramatic dv addition prograde to end up near orbital. The explosion seems pretty undirected, so the force will be spread in all directions, so most of the force won’t be prograde. Even still, it can’t be orbital. It’d need some other force added later on its trajectory or it eventually had to pass through the same altitude that the explosion happened, which is in-atmosphere and will cause fairly quick decay. There is no risk of Kessler syndrome. It could potentially have posed a risk to some very low satellites, but we’d already have seen that happen by now if it were going to.