That's the specification for source current taken directly out of the 32u4 Datasheet and I was writing under the (possibly now erroneous) assumption that sink current capability would be the same as source current. Of course one would never push GPIO pins to that extent anyway. However, now that you mention it, I do have a vague recollection that someone might have mentioned quite some time ago that sink does not necessarily equal the source current despite what some references might say. If so then I stand corrected (maybe again).