In ‘Rado, they like to attach Roubaix to the name of gravel events. The one in Boulder is long standing and a sanctioned event. I’ve done it in the past on 23s. 25 would have worked but you’d be getting dropped instantly on a gravel bike.
Im signed up for 3 other Roubaix rides this summer. Two of them, I’ll take the road bike. It’s like the real one, simply too fast of a course with too much road to justify 40mm tires and a heavier bike.