Its not clever to ride two abreast, but its perfectly lawful.
They haven't committed a moving traffic offence, he wasn't stopping them to inspect their documents (because cyclists don't have any), and they weren't wanted on suspicion of committing a crime, so I'm not sure the stop was even lawful.
Not suspected of a crime or ASB, so I would question under what power he requested their details.
Other drivers drive carelessly of their own volition, and are solely responsible for their own behaviour. If he'd stopped me I think I'd have had him in front of a disciplinary panel unless a sincere apology was forthcoming.
Traffic were godawful. Spent a few years there myself and didn't enjoy it. When I was in CID the Ice Cream Boys would often bring in some cracking collars, and then muff it up evidentially because of their lack of knowledge of law and procedure. A good case for fixed tenure if never there was.