the sorting actually happens in `_factorize_keys` [a few lines down](https://github.com/pandas-dev/pandas/pull/27762/files#diff-14fc0a514ad9ce2a73eeaa9ec3fb9406R1292), not downstream in `_right_outer_merge`. So the left and right keys need to be swapped before that call. `_right_outer_join` just implements `_left_outer_join` with left and right swapped; no sorting happens there.