The white pudding recipe linked also specifies FRESH suet, not RENDERED suet, which means there is more then just the fat in it. But as it boils, the fat expands the starch granules of the oatmeal, cooks the onion, and some just disappears into the cooking water.

the end result is admittedly high fat; but it is a mix of cracklings, oatmeal and onion.