From the answer in the other question, that user does not have an associated account on the migration-target site.
However, voting activity is associated with posts. So, if and when the user's unregistered account is associated/merged, all the posts get associated as well. At that point, they will get a rep recalc, which will let the real reputation score show up.
So, the voting activity counts and is valid, but the score will only show up (along with a full user profile) if and when the user's accounts are associated.