It's not a "rule", just a recognition of reality. The job is done when the job is done. The pass is completed when the guy you're passing can no longer prevent you passing. There are too many rules about passing. It's to the credit of the stewards that they decided not to get involved in this incident.
Clearly it was not just in his head, since after the contact it was Nico who ended up off track and losing places, not Lewis.