I suspect the EXE description at your link was not carefully worded.
Perhaps it works the opposite way round from what you stated (and what was stated at your EXE documentation link). Instead of
wenn bei Offset 12h/13h im EXE-Header der Wert 0 steht, erfolgt keine Überprüfung der Prüfsumme.
my guess is that it is
wenn keine Überprüfung der Prüfsumme, steht bei Offset 12h/13h im EXE-Header der Wert 0
But this new statement no longer means that zero's at 12h/13h indicate no checksum was computed.
So if you know that your EXE was published with a checksum, then even a zero value in 12h/13h has meaning as a checksum.
Jack Gatrost
Software Development
Beck IPC GmbH