Started a game recently with the alt-champion progress and alt-Nemesis response, along with the new golems, to see how it all played out. Beat one of the homeworlds, colonized it after killing the command station, and started marching most of my fleet towards the other homeworld at the other side of the galaxy at the speed of an Artillery Golem.
Between my fleet leaving the first homeworld and arriving near the second, a nemesis exo triggered. The attached screenshot shows what happened
They're all the AI version of the shadow frigates, and they started on a move order from what used to be the homeworld, but I can fly them around and kill AI ships with them just fine. They can place the champion shields, but it uses the same store of total energy as my regular champion.
A second wave of (actual) AI champions spawned at the second homeworld as normal, and ran into my fleet.
I'm guessing this is either a hidden feature, illustrating clever humans capturing the source of AI Nemesis frigates, or a bug. But if noone feels like patching this 'bug' right away, we can probably find a way to live with it
(save attached in case it helps anything)