Because 'defective', 'knackered', 'faulty' etc. are very generic terms to describe something that isn't functioning as it should. In reality there could be any number of faults with your battery and each would give a different symptom. We put them all under one heading, 'defective', and you could say that's all we need to know. The battery is either servicable or it isn't.

An electronic control unit however will see things very differently. A battery could have an open circuit fault with a cell and the ECM would respond accordingly. Equally the battery could have a short circuit cell. Both of these faults would to us mean the battery needs replacing, but the ECM would interpret them differently because that's what they are.

The answer to your question is therefore simple. Your car's engine control module receives input information from sensors which monitor everything it needs to know, including battery voltage. Your old 'defective' battery voltage was unstable, caused by whatever was actually wrong with it, and the ECM responded accordingly.

Maybe refreshing the battery connections helped a bit?

Seen this and similar on a few cars,

Jag was mentioned,

BM's and Mercs.

Low battery voltage can cause some very confusing performance issues

and I use the term battery voltage because there's some confusion about that,

the one sure thing is IT'S NOT 12 VOLTS!

If yours is showing that or less the battery is shot.

worse would be the battery having a massive internal resistance to charge.

which would see the alternator trying to do something like force a golf ball down a garden hose.

If you were to unplug this alternator then start the engine then re plug in the alternator you would hear/see a significatnt rpm drop,

an alternator is an electro MAGNETIC device.

And if you battery has a high internal resistance trying the previous test it's possible the engine would stall from the increse of restance coming from the alternator,

or as it's functioning now as an electro magnetic BRAKE!

Totally plausible.