Safety of a mass driver tug, or lack thereof

I have seen statements of the form "The pellets ejected from a mass driver tug would not measurably alter the number of debris objects in near-Earth space" or words to that effect.
Yes, that was the conclusion of the original NASA Space Settlement study.
Perhaps I just need to really understand more deeply, but those words are not very comforting to me. In my mind, pellets ejected from a mass driver tug would go into various orbits around the Sun, more specifically, orbits that intersect with Earth orbit or with Luna's orbit.
True, butthe Earthwould most typically not be at that point in the orbit at the same time the debris was crossing the orbit.
An important point is that the use of pulverized Shuttle ET pellets, raw lunar or asteroidal regolith, or slag for reaction mass was only envisioned for the very earliest days of space manufacturing. Once ore refining operations are underway, we would be producing more oxygen than what's needed for atmospheres or for rocket oxidizer, and would then begin using that for reaction mass in mass driver tugs. As you correctly state, oxygen would vaporize and thus pose no hazard.
Can you make me feel more secure about firing off pellets of slag from a mass driver tug? Do you suggest we track the orbits of each pellet fired, and propagate each of those forward for! the next 100 years?
I'd agree that's not apractical proposal. Like I said, we would not engage in the use of solid reaction mass for a prolonged period, and thus would make only an insignificant contribution to an already existing hazard.