Torque Setting Of A Compression Nut
The steerer-tube plug or bung has two functions:

Provide a backstop for tightening the stack;
Resist the crushing force on the steerer tube from the stem pinching it.

So the plug needs to be tightened just enough to serve the first function (for which some friction paste may help); this will naturally put it in a position to serve the second function.
It's also difficult to torque down hard on the plug, as there's nothing except friction against the steerer tube to hold the body in place as you tighten the bolt.