Let me see if I have this right: when the bolt head is screwed tight up against the shoulders on the head and bolt body, the recoil forces are transferred through the shoulders.

If the bolt head is so loose that when in the closed position in the rifle the shoulders do not touch even when the bolt head is pushed backward by recoil, then the recoil forces would transfer through the threads.

There must exist a few thou of clearance between the individual male and female threads, so when the bolt and bolt head shoulders are done up tight against each other, the thread faces that would transfer the recoil forces were the bolt head to have gross overturn, are actually not in contact with each other at all.