You're conflating throwing a flag for a foul, and seeing something likely across the field, in your vision/zone (that isn't technically your direct responsibility). In this specific scenario, multiple officials had the opportunity to see the action around the completed/incomplete catch, and then if the safety had a brick in his hand, coming in hot. H, S, and B should all have had this in their direct view, and F definitely should be aware of what is happening on the other side of the field, and if he's paying attention, most likely would be able to see the safety screaming in for a big hit, which should hold his attention. Taking information - and opinions on what transpired - from all the officials who might have a relevant piece of information, is how we get the hard/complex calls right.