That article also contains a link to another of their sandwich articles which includes this
QuoteDonkey Burger, China
In the Chinese cities of Baoding and Hejian, buns stuffed with shredded donkey meat, sometimes referred to as “donkey burgers,” grace menus at fine-dining restaurants and street-side stalls alike. When it’s marinated and cooked for hours, donkey meat turns from sinewy to tender and sweet. Some cooks mix chunks of fat or green peppers into the naturally lean meat, creating a uniquely textured, flavorful sandwich. All that gets stuffed into warm, flaky buns—round in Baoding and rectangular in Heijan.
I don't know where they buy their donkey meat or sandwiches, but I regularly eat donkey in various ways and various cuts and have never encountered 'sinewy' meat. Quite the reverse. It is the tenderest red meat I know. A local restaurant does the sandwiches which I often eat.
