@jens @sotolf @slothrop @schratze @elexia
like all dictionaries, the Duden is a record of how people speak, not a manual of how to speak. it's fieldwork, not a book of manners. if a word is in Duden with gender blah, it means they found that word in their corpus (currently at some 6.7B items) being used with gender blah. not that some language God-King has decided on first principles that the gender shalt be blah.
that's why it has all of die, das, der for Cap, in this order; it means "we found out people are using all three genders for Cap, in this order of frequency". you will notice that my little Mastodon poll reproduced the same statistical results (though people's self-perception when asked is always less reliable than a statistical observation of the language they produce when you're not looking, including our own).
that also goes to stylistic labels like "umgangssprachlich". from Duden's own documentation,
> Was manchen Benutzern normalsprachlich – weil dem eigenen vertrauten Lebens- und Sprachalltag entstammend – erscheint, ist für andere schon „umgangssprachlich... Angaben zum Sprachstil, zur Sprachebene, sind immer wertend und damit oft subjektiv. Dies gilt bis zu einem gewissen Grad auch für die Angaben in Duden online, obgleich sie sich auf eine Fülle statistisch ausgewerteten Materials berufen können.
you will notice that people in this very thread were describing "der Cap" with emotionally-laden pejoratives, despite "der Cap" being present in what you just called "the bible". that is the kind of thing it means when a form is marked "standard" or "proscribed" in a dictionary. it does not mean "the language God-King has decreed thou shalt not use this form". it means "although this form occurs in our observations, some speakers show bigotry if you use it".
(the one exception is orthography, since writing is not really language but an artificial method of recording language, and therefore unlike language can be planned by someone who makes up the rules. then, yes, a dictionary like Duden acts like a repository of examples of how these rules are applied, and can make suggestions (in Duden's own words, „Empfehlungen“) about tricky or ambiguous cases, based on first principles. this is a consequence of writing being a second-order notation system. something linguistic proper like syntax, morphology or phonetics works differently; the patterns are not designed, they are discovered.)