Or she just got through the screen tests and in-tent baking trials OK, but just couldn't cope well with the actual pressures of filming? From what previous contestants have said, even with the desire to cast for a variety of broadly appealing 'types' (which has been there since early seasons), potential contestants are still subjected to rounds of interviews with questions about what exactly they bake at home, the specifics of baking techniques etc, before they get to the screentest stage.
I can't imagine the GBBO casting director saying 'Well, she can't bake at all, but she'll be excellent TV!' Though one imagines they will have some idea in advance who's likely to survive till the later rounds...?
Also, if Danni told them she was vegan and wouldn't be tasting anything she baked in the tent that wasn't, and had sent in (well in advance, as required) a non-vegan cookie recipe, then I think that's at least partly on the production team for not saying 'No, you can't bake stuff in the tent that you're not tasting and have never been able to taste when practicing!'
I haven't watched GBBO much in years, but what's the position on vegan bakers before now? Surely the technicals are overwhelmingly not vegan, so they must previously have had vegan bakers who couldn't taste as they were going along, even if they only made vegan recipes when left to themselves?