Twentieth-century campaigns against religion in Mongolia were directed at institutions and at conspicuous objects. Monasteries were destroyed, monks were killed or laicised, images and texts were burned, and the visible apparatus of practice was very effectively removed. A drum, a costume or a standard is a thing an official can identify, confiscate and account for.
A wooden spoon is not. The tsatsal is small, cheap, domestic and indistinguishable at a glance from ordinary kitchen equipment, and the gesture it serves takes two seconds at the door of a dwelling before anyone else is awake. It is in the nature of that kind of object to persist through periods when nothing else can, in households where no other practice continued and where the people doing it might not have described it as religion at all.
This is a general pattern rather than a Mongolian peculiarity, and it is worth stating because accounts written from the outside consistently underestimate it. Surveys of what survived a suppression tend to count institutions, buildings and specialists, all of which are visible, and to miss the domestic layer entirely. When practice becomes possible again the domestic layer is usually what it is rebuilt from, and in Mongolia the milk offering was one of the things still there to build on.