The mob rule does not by default make them fearless. It lets them use a Leadership equal to the number of orks in the mob who have the rule. Additionally, if there are 11 or more orks, they are fearless.
Since your leadership as a value cannot be over 10, for any test they are required to take that fearless does not exempt them from, they use the # of boys OR the max LD of 10. If a weirdboy is in a mob w/ 11 orks, he's fearless and cannot be pinned. ALSO, his LD = 11+, which reverts to the max of 10. This makes sense fluff-wise also, for w/e that is worth, b/c the psychic power of the weirdboy and the ease with which he can master those powers is directly related to the # of orks nearby amplifying it.
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