How so? There, n means "the number of [this type of] drivers", thus the GP is saying that "self driving cars improve in a way proportional to the number of self

How so? There, n means "the number of [this type of] drivers", thus the GP is saying that "self driving cars improve in a way proportional to the number of self driving cars [that are collecting and sharing data], while humans improve their driving skills in a way independent of the number of humans driving." (I think that's slightly wrong -- humans are probably O(log n) because large populations of humans invest in heuristics and teaching methods that enhance driver improvement rate, and for SDCs, the "lessons learned" between units are probably somewhat redundant, so it might scale as something like sqrt(n). But I don't see a problem with the O(n) usage.)