Your 'standard' ambulance, the kind with the 6-minute response time, is staffed by two EMT-Bs (B for Basic).
The treatment that can be provided by an EMT-B is...basic. It's mostly limited to CPR, oxygen, splinting, bandaging, and administering the patient's own epi-pens, inhalers, and nitroglycerine. These are all valuable interventions, but they're 'first aid,' not 'treatment.' Nearly all of them can also be provided by a certified first responder.
The people who can actually treat patients are AEMTs (A for Advanced) and EMT-Ps (P for Paramedic). They have longer response times because there are fewer of them. Each medic unit has to cover a larger area than a basic ambulance.
For potentially time-critical calls (cardiac arrest, drowning, choking, shooting, stabbing, unresponsive, head injury, etc.) the goal is to get someone on the scene as soon as possible. Once someone's there, they can actually look at the patient and decide whether the higher-level (and slower) responders are actually needed.
The way 911 dispatch handles these calls is is that they immediately dispatch both a basic ambulance and an AEMT or paramedic unit. If the ambulance response time is looking slow, they'll also dispatch the nearest fire engine. And if an engine crew or other first responder unit (police often listen to fire department radio) happens to be nearby when the call goes out, they'll often radio in and request to respond.
Now, if Uber can put a CFR on the scene more than twice as fast as an ambulance, that's a 100% positive thing. They can cancel unnecessary ambulances and medics. They can get stable, low-priority patients to the hospital faster and more cheaply than an ambulance. And if a patient is in serious distress, they can begin administering CPR or first aid several minutes faster than would otherwise have been possible, which will save lives.
It might also make sense to dispatch Uber alone (with the caller's consent) to calls that are clearly low-priority. A fair number of 911 callers are alert, ambulatory patients who just need a ride to the hospital. A private vehicle could get them there faster and cheaper, but low-income patients often can't pay upfront for a taxi. They're stuck calling an ambulance, which forces the public to absorb the expense. If you can put them in an Uber instead, everyone wins.