Drop bar road bikes are safer in a group of road bikes closely following each other. The drop bar isn't going to snag anyone passing closely, nor impale anyone in a crash, and the braking characteristics of a MTB with MTB tires versus a road bike are... um, different enough that it's a problem in a closely following group.
As for slowing the group down or not having skills for a group ride, it's a reasonable heuristic. Making judgements based on probability is... well, actually, the basic method of most everything which isn't physics. Of course, you might be the exception but their group ride is, after all, about recreation, sport and relaxation for them, and filtering those who are unlikely to fit with the group makes sense. Especially because your bike handling skills in a group are a potential safety issue for them.
(By the way, the reason nobody uses drop bars in MTB races is because drop bars are banned by the UCI for MTB races. Preserving the traditional look of the sport. Surprising that people don't know that.)
Last edited by Branko D; 11-08-21 at 09:22 AM.