It's a legitimate practice in science to speculate. Having heard the Harvard guy explain more fully the Oumuamua thing, it's struck me as perfectly fine activity for some scientist to look into. His hypothesis is almost certainly going to be untrue, but it's fine to investigate a bit of a moonshot idea. You don't want half the field doing this, but you absolutely need different little pockets of speculative work going on in order to keep scientific inquiry open, dynamic, and diverse.