https://dizionari.repubblica.it/Inglese-Italiano/D/default.h...
Nowadays it is not used much, but it has been for years in texts related to IT, a reference:
http://vietatoridere.blogspot.com/2012/06/default-significat...
BTW "per difetto" is also used in legal matters to mean "in mancanza"
Take a look at this http://www.treccani.it/vocabolario/difetto/
The only use of "per difetto" that existed before a translation of "by default" was ever needed is in the expression "approssimazione per difetto", and the new meaning forcefully attached to "per difetto" makes sense only in the worst nightmares after a "peperonata co' e cozze".
Picchi probably is only witness of a niche neologism, that was probably bred from a bad technical translation. But it was a bad neologism in the first place, it was incorrect right from the start.
EDIT Nevertheless I take your point: you did your research before you wrote "correctly".
It's not something ridiculous out of left field.
No, as said it is used in legal matters, example:
https://www.brocardi.it/massimario/59534.html
And you go tell this Italian professor:
http://www.diit.unict.it/users/alongheu/linguaggi/aa0910/lez...
The point is that it is actually correct in Italian, if you think about it also in "arrotondamento per difetto" (as opposed to arrotondamento per eccesso")[1] the meaning is that of "arrotondamento per mancanza", i.e. meaning #1 in the Treccani dictionary while it is much more commonly perceived as meaning #2.
[1] respectively "round down" and "round up"
At first sight this (default = difetto) looked like so many careless anglicisms, where a faint similarity in sound and meaning induces someone into believing it is a translation.
And this happens really more than needed in technical texts. My point was that you cannot trust technical people to make good translations, an it is outrageous that a translation is considered correct just because it was written in a book. But this does not apply here.
I am making up my mind that this was instead a delicate and careful intellectual construction that dutifully just failed to convey the meaning.
So be it.
Actually, I believe the etymological root of "default" is exactly the same as "difetto."
"from Old French defaute (12c.) "fault, defect, failure, culpability, lack, privation," from Vulgar Latin *defallita "a deficiency or failure,"" [1]
The issue is that in English this was then used to mean "failure to pay a loan" (in 1850s) and then someone in the 1960s started using it in computing to mean the option chosen if the user fails chose."
So English just stretched the meaning way beyond the original, so it's no surprise that this new meaning doesn't really match in other languages' cognates for the word.