That’s not what the article says.
> Throughout the war, stretches of the Western Front observed an unofficial “live and let live” policy between Germans and their French or British enemies. By mutual agreement, both sides agreed not to attack the other unless ordered — and would even schedule truces for meals and bathroom breaks.
The German that shot him was probably just messing around, you're right.
It's far worse to uncritically examine our ancestors behaviour.
Your link also states "Please respond to the strongest plausible interpretation"
Both their parent and GP were talking about reading conflicting things. Reading around to get a better sense of the truth of the matter would seem, reasonable.
Of course a less pithy comment, where the meaning was clear would be best of all.
It’s likely your grandfather was a good man with no desire to kill Germans. It’s likely whoever shot him felt the same.
No doubt they were. But we do tend to forget that large swathes of them had originally volunteered, often with enthusiasm.