Studies of the flail arm variant, which my husband has, reveal that some cases disseminate to the rest of the body and others do not. An EMG demonstrating abnormal recruitment/motor units in limbs or regions other than the arms would suggest the former, as was my husband's case. It is also important to note that "median" means half above and half below the measure reported, so half the FA group had lived longer than 57 months at the time the data were analyzed.
That said, the case series vary in terms of criteria used. One working definition is arms-only symptoms for 18 months or more. Obviously that "stacks the survival deck" in studies.
Hope for the best (progression stays limited to arms) and plan for the worst ("standard" ALS).