Maybe it thought they were on a bike lane? Where I'm from it's illegal to ride like that in a group outside of police-sanctioned events - you're supposed to ride one after the other and keep right.
I’m in Massachusetts where lane splitting is legal in some cases (single lane each direction, posted speed limit below some amount, probably something else). Some other states have similar rules, but in most it’s flatly illegal.
I believe California is an unusual outlier in that lane splitting is allowed with no additional restrictions.
> you're supposed to ride one after the other and keep right.
In many states bicycles are entitled to use the full lane. I often feel safer riding in the center of the lane for a few reasons:
1. The center of the lane is less likely to have small ride-ruining debris like glass, sticks, and nails
2. Cars surprisingly give me more space when passing if I’m not riding off to the side
3. It seems that potholes occur more often to the side of the rode than in the center, or at least get fixed more often when they’re toward the middle of the road
Note: I’m talking about the middle of the lane vs the side, not the middle of the lane vs the shoulder. The shoulder isn’t something you should travel in anyway, even on a bike.