Not even elders are allowed speak to (make small talk with) DFed persons.
Only elders who are authorized by the whole body of elders may speak to DFed persons once a year to see if they want to return (to their vomit).
And for those DFed who visit meetings, elders aren't allowed to talk to them unless it's necessary for their reinstatement or if the person needs to have some arrangement for transport to meetings.
So the CO just did as he and other elders are supposed to do: no make small talk with DFed people.
By quickly stopping to speak to the DFed, the CO also gave a strong signal to elders: Follow the rules! Shun! I listened to this minor thing you brought to my attention, and in reciprocation you should obey my every word!
Him coming back to say 'sorry, wont talk to you no mo' is probably his last shred of human decency combined with the desire to let the DFed know they are being punished because they're DFed. He prefers to be a shunning asshole over being mistaken for an asshole who forgets about a conversation and doesn't return to it.