The effects from Tenth District Hero's abilities overwrite other effects that set power and/or toughness if and only if those effects existed before the ability resolved
Though we dont usually see more recent cards, particularly of less popular characters such as Gengar, reach such heady heights, theres a number of reasons that make the situation clearer in terms of this cards value
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No regrets, because it was a heck of a lot of fun playing Magic: The Gathering in the food court back in the day
Bag of Devouring Bone Devourer Book Devourer Devour Flesh Devour Intellect Devour in Flames Devour in Shadow Devourer of Destiny Devourer of Memory Devouring Deep Devouring Greed Devouring Hellion Devouring Light Devouring Rage Devouring Strossus Devouring Sugarmaw Devouring Sugarmaw Devouring Swarm Devouring Tendrils Dream Devourer Effluence Devourer Faceless Devourer Galactus, Devourer of Worlds Grothama, All-Devouring Hidetsugu, Devouring Chaos Intellect Devourer Morlun, Devourer of Spiders Nim Devourer Phyrexian Devourer Shriekwood Devourer Silvar, Devourer of the Free Thallid Devourer Thassa's Devourer Thought Devourer Venom, Deadly Devourer
Think of it as a contract between MTG players and Wizards of the Coast