The reason was to "preserve their neutrality", but the US should have incinerated them.
America's 'War' Against Switzerland
https://www.youtube.com/watch?v=FILxoQyKzDg
I've talked to Europeans in person, and they have the strange idea that tweaking the US military has no repercussions.
I'm not sure what this has to do with the linked story, though.
Regarding the 20mm shells, German fighters had a 20mm cannon that fired through the propeller hub. These were not big bullets, but were packed with explosives [1].
The linked story seems a little bit "too good to be true". A tall tale.
WW2 is not the 'cold war' where every little visual sighting counts as 'an encounter of note'.
In a world wide warzone, this would be the the tip of the iceberg in terms of aggression, borders, politics, unrecorded deaths, unofficial actions, backroom deals etc..
"I've talked to Europeans in person," I'm not sure that having spoken to a 'real live European(!)' counts for any kind of credibility.