It appears the current version of Drupal outside the issue fork needs to be added as a remote repository, but i'm getting tripped up on creating it as a branch i can rebase against.
Hoping the answer to this is a link to documentation, but https://www.drupal.org/docs/develop/git/using-git-to-contribute-to-drupal/rerolling-patches does not mention the issue fork workflow.
This seems to do it, where '3029545' is the issue number which is forked, these steps are presumed to have happened first:
git clone [email protected]:issue/drupal-3029545.git
cd drupal-3029545
And now rebasing on most recent Drupal core:
git checkout 3029545-add-route-exists # check out the branch in development
git remote add drupal [email protected]:project/drupal.git
git fetch drupal
git rebase drupal/9.2.x # current core branch