Seat Ibiza stopped working after battery replacement.

Hello!

Okay, here's the translation:

Mal eine Frage in die Runde der Fachleute:

An elderly gentleman from our village had left the lights on in his Seat Ibiza, which was about ~ 12 years old. This is a typical retiree's car, a small gasoline engine, and I don't have any more details at this time.

Anyway, yesterday the battery was completely empty, my father removed the battery, and we charged it overnight at our house.

This morning, he set off confidently, having installed the battery.
But the car has only reached a speed of 2000 RPM.
Since I was at work, the ADAC tried to fix it, and after using their diagnostic tool, it was working again.

{QUESTION}
What could that have been?
So, I know how to train the throttle, after removing the battery without a buffer battery,
but such an extreme case, where the car is practically no longer roadworthy, has not occurred to me before.