Umm Salamah's question focused on Ibn Umm Maktum being unable to see them. The Prophet's reply also draws attention to the observer's own responsibility for looking. Modesty is thus more than simply avoiding being seen by someone else; it concerns one's own behavior.
The incident occurred in a particular gathering with particular addressees. It should not by itself be made into a comprehensive ruling for every circumstance; detailed legal applications require separate treatment.
A sound practical lesson is to respect people's dignity, maintain appropriate boundaries, and behave with restraint regardless of whether someone else can see us.