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A neutral creature should generally be stronger than a standard unit, but this one is not only moderately strong physically, but also noticeably weaker. And there will be twice as few of them. When they appear, their "sign" (+/-) deals little damage, just like a shot, and it seems that shooting might even deal more damage – I'm not sure how the ranged attack penalty works in melee combat here. If it couldn't shoot at all, but could endlessly apply the "sign of fire," that would be a completely different creature, with a purpose as a ranged attacker, and even with these stats, it would be more desirable than a gremlin. Here, even a downgraded gremlin will be better simply because there are twice as many of them.
And if you look at a battle for a mine, for example, where they are on their own without a company of fire mages, their "sign of fire" is completely useless as a debuff.
Some points can be explained by the description:
Fire ammunition is a rare thing; there are few factories, and few gremlins who specialize in arson, and they are not used in cities; the mechanics are more useful in the army of golems... that's all I can explain