An overcurrent fuse by itself would not prevent the battery from heating a low resistance (not dead short) path
True, also it provides no protection if something was trying to 'charge' a non-rechargable cell but I really can't believe Honeywell won't have thought of that and ensured that even if volts were applied where they shouldn't be, no reverse current can reach the battery. If it is the first ever 'spontaneous combustion' of this ELT Boeing must be thinking someone hates them or the planets are very badly aligned right now, some reports say over 6000 of this type are out there!