I just got a response from the Siglent EU support:
"thank you for the information.
In the meantime, we found out that we forgot to adapt the documents to the function.
The complete SDM3 series is working actually with 10% overload.
Sorry, for the mistake in the documentation.
My colleagues in the head quarter will change that as soon as possible.
For the moment there is no way to get the 20% overload with your device.
Sorry, again."
So I think, that's it, case closed
bummer. I don't really understand what has to be adapted, as it never did 240 000 counts, so the documentation has been wrong for like 8 years, but whatever.