I mean apparently Ethniu and the Fomor invading Chicago was a big diversion, with the real goal being putting Thomas in Demonreach so that Harry would let Nemesis inside.
For that to work you need Harry to beat her. (It also requires the svartalves to torture him and not question him even though he's obviously just the tool in this scenario, which is kinda weird for them) so it's possible that Nemesis had more then one agent in play here.
Or that it can just predict the future. At least one of the two is required for a plan this convoluted to conceivably work.