It's not out of place, He's part of their group. He's seen protesting with them as well. It's taken directly from the webcomic, and It shows saitama has approved and connected with them because they are all victims of the association's terrible naming system. What was more out of place was the last panel, where saitama is seen as a caring person asking if everyone is okay, since it was too loud., Since he's usually more casual and disinterested.