The standard is:
Redundant gas source - pony cylinder, twin cylinders with isolation valve or sidemount configuration. Redundant gas supplies must be configured so that the diver can access it with one hand.
From my perspective, a Spare Air is NOT a pony cylinder.
The course theory is detailed on gas management. It gives clear in sorry on how to "establish the cylinder size needed for a planned dive".
It is also a skill performance standard (dive #2) to:
"While continuously swimming, simulate an out-of-air emergency and change from your primary air supply to your redundant air supply system within 30 seconds, then breath from the redundant air supply system for at least TWO minutes"
Good luck for 2 minutes at any reasonable depth on a Spare Air..
On dive #3, the performance requirement is:
"While continuously swimming, simulate an out-of-air emergency and change from your primary air supply to your redundant air supply within 30 seconds, then deploy a lift bag or DSMB and ascend to the surface, stopping at 5 metres/15 feet for a three minute safety stop".
Sadly, the manual provides no direction or standard on the minimum capacity for a redundant air source. It gives gas management info, then merely states that "As a self-reliant diver, your redundant air supply system covers you for low on, or out of gas problems".
I doubt PADI had a spare air in mind. But, with no direction on min volume, it's open to interpretation of what a 'pony cylinder' is.
Only a dick instructor would allow a Spare Air as redundancy IMHO.