Weak and not eating is a "Failure to Thrive" Not even an EMS call, but a scheduled transport type call, but since they called 9-1-1 they're getting the EMS response for an illness.
On a callback, or if the calltaker was still on the phone, and the "incoherent" part was learned, the call is then upgraded to an "Altered Mental Status" type call, which could be from several causes...Alcohol, Epilepsy, Diabetes, overdose, underdose, trauma, infection, psychiatric, stroke...and is of a higher priority, one which dictates the response of an engine company. Since the ambulance isn't onscene, they have to start the response to ensure that the proper response is enroute. Since the ambulance was already on the way, they got there first, went in, saw the patient and canceled the engine company since they determined it wasn't needed. The proper response was initiated both at the time of the original call, and at the time of the additional information. Yeah, they probably could have gotten away with not sending the engine, but my guess is that they really don't know where the SEMS unit is coming from, so it could still be a shorter ETA for the engine company.