They weren't riding as close together as the pic, I would say the pic below is 4 abreast but open to correction!.
Hey, look! A motorist able to overtake "4 abreast" (and I think it's 3, appearing as 4 due to foreshortening) on one of those narrow "single lane" roads while allowing a whole car's width of space! If I hadn't identified it as a picture of 303cycling.com (in Colorado), I would have guessed it was Canada
The
article containing that picture is titled "What To Do When Your Cycling Group Ride Becomes A Traffic Problem" and is chock-full of typical advice from the Motoring-supremacist States of America, starting from an attitude that cycling is not traffic or of an equal status to motorists, should "stay to the side of the road" and show "respect for traffic" (oh crikey,
not that old BS again) and there's this gem by the author in the comments: "The solution of taking half the lane, riding about two abreast, may make the line longer but a vehicle can pass completely in the other lane and if needed, can slide right"

Yes, that's exactly the reason
why groups take the lane, to discourage motorists "sliding right" in the face of an oncoming motorist after a misjudged overtake and squeezing cyclists against the edge of the road (or off of it).
Despite the name "I Love Bicycling" doesn't seem to love riders! We must not allow this fair isle to go the same wrong way as America!