It's not about Tribute (when this card shows up the "tribute for level 5+ monsters" rule is already in place)
It's the power level at that time. Back then the highest ATK for a Level 4 or lower monsters is 1300 iirc, hence why anything with 1500+ ATK are Level 5 or 6.
Actually, Doma was indeed useless at the time of its release. 1500 ATK Level 4's were common at that point (Indeed, there were 4 in the same set as Doma.) And the previous Volume had Skull Red Bird and Rogue Doll, even stronger Level 4's. There were also stronger Tribute Monsters at that point. (Mostly in the 1650-1750 range, though Summoned Skull and Curse of Dragon were things too.)
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