I would like a lot more corroborating evidence before accepting them as fact.
So you also don't accept 95% of the content in 1st-hand accounts of combat in all those books you read, because the large majority of material in personal accounts are not corroborated by a second source (or more than a 2nd source).
Things like a rare pilot here or there using flaps in combat you disbelieve, because while you might have seen a couple of mentions of such flap use in memoirs, for each such mention there are 50 other personal accounts that don't mention using flaps in combat and even a couple of personal accounts where the author says "we didn't use flaps in combat".
Zemke's account of surviving the disintegration of his P-51 in a severe storm, you would disbelieve.
Steve Pisanos's story of surviving a crash landing where he was out on the wing of his plane when it hit the ground, you would disbelieve.
Smith's story of his B-17 being saved by two P-38's where one P-38 dove on a formation of about 50 Bf 109's and chased them off, you would disbelieve.
Galland's story of flying alone to the English coast for a bit of combat with lobsters and champagne in his plane, then coming back for a friend's birthday party, you would disbelieve.
Hartmann's account of how he escaped Soviet captors you would disbelieve.
The only stories you would accept are those where there is a 1st-hand statement, and a researcher digs into it to find a 2nd person who claims he saw it and it is true, or (since I already did that with the "fork-tailed devil" issue -- two stories that say the Germans used the term) maybe 3, 4, or more independent references that say the same thing.
Do you see that a vanishingly small amount of the material in war memoirs you've read rises to that level of scrutiny?
Even the fact that in some German publications the phrase is repeated using bad grammar indicates that the term originates from a person who's first language wasn't German.
When it comes down to a disagreement between you and professional staff of German publications on how to write a German phrase (in works that are entirely German with apparently no scrap of English in them, by the way), I'm sure you would not be surprised that I believe the later to be more of an authority on the issue.