It's too expensive to buy one just to tear it apart or even characterize it and return it(*), but if it's a (20V) zener or equivalent, I'd be concerned about the long-term dissipation (does it heat up if you're running the alternator into just the APM?) and we still haven't solved the problem of using the BMS for charge regulation.
A drop-in starter battery would be more complex, and hence more expensive, than the battery manufacturers want to create, would need a custom BMS, an APM function, etc, and everyone's aiming for the low price point, not the perfect functionality and 8000-cycle 10-year warranty.
Good, Fast, Cheap, pick one?
(*) I don't do that kind of thing.