That looks to me very much like an explicit statement that the Nemfection proceeded Athame --> Lea --> Maeve.
But it doesn't say how, does it?
That's how Leanansidhe was tainted--and your godmother spread it to Maeve
It implies that Nemesis is catching like the measles, but we know it isn't that simple.
When Harry says to Mab that it must have hurt her to see her daughter infested with Nemesis..
She answers;
"It was the Knife."
The Knife was the original source of the infection, implying like bad potato salad at a party, those who ate it got food poisoning. Lea may have brought the salad to the party, ate some and offered Maeve a spoon of it, so both got sick, but again not that simple.
On the other hand, the fae do love their tricksy language, implying something without overtly stating it as a fact.
But I think Mab would have needed an objective, here -- why would she want to mislead Harry about this?
I don't think she was, or rather she may have not wanted him to know that she herself was in danger of infection, lets not forget she wore it for at least one scene in her belt. I believe she, herself was taking the cure in Proven Guilty... Which makes sense since both she and Lea had direct contact with the infected Knife. However if Maeve didn't have direct contact with the Knife and still got infected, something else is going on. Also at some point Cat Sith was also infected, how did that happen? Which could very well be the case as you say, then Mab missed it because she assumed that direct contact was needed.. That's why her anger in the chapel back in Small Favor, she was fooled and her daughter paid the price. Or she wants Harry to get to the bottom of it, but because it is Nemesis won't come out and say it, that would be very Mab-like..
Back to the original question, so what happened to this tainted object?