From what I recall, Animus works in the following way.
X has an ancestor, Y. X and Y share genetic material. X can access Y's memory through the Animus simulation.
In Assassin's Creed Valhalla, near the endgame after the "Brother’s Keeper" mission, a certain person uses the Animus and simulates a memory of Eivor.
The original user has genes common with Eivor, being his/her descendant, but the new user does not. How is that possible?