These are... nespole. I have a tree in my garden and we, in Italy, commonly eat them. A bit disappointed after the intriguing opening of the article.
As we grow a lot of historic varieties of vegetables a "Mispel" was not so unusual for us.
Perhaps it's no coincidence that the article doesn't mention Germany at all, it might ruin the "forgotten" hook. (no you can't walk into a German supermarket and walk out with a bag full of Mispels, it's not that un-forgotten)
This is the supplier, basically the "industry giant" amongst the traditional distillers of the region (most others aren't full time occupations I think): https://www.destillerie-haas.com/epages/61827331.mobile/de_D...