This. You could add them back and approach them in a private plurk, but sometimes people are reluctant to do that because it could be putting pressure on someone who's nursing some bad feelings.

Some people also do a cut like 30 minutes after doing the announcement, which is a bad idea. It may be that they just did a group cut and said that if they cut anyone who wanted to stay that person should just add them back. You can't see that if you weren't looking at your timeline in the small window of time that they left for you.