Differences in CMOS fan-out between the two logic level
That may have been true at some point, or it may not have been a CMOS output.
CMOS outputs typically use symmetrical output strength, but it means that the weaker PMOS transistor can be made larger than the stronger NMOS transistor to compensate the strength.
LS TTL outputs do have asymmetrical drive strength, but LS TTL inputs also have asymmetrical input drive current, so they also match up.