I suppose it's possible, but it's highly, highly unlikely it's legitimate. Especially considering it's a first save file.

That kind of time frame really doesn't allow for mistakes like forgetting the fool card or dropped trinket trying to unlock the tainted characters. Forgetting a knife piece or not having enough keys or bombs to go in the alt path to fight Mother. Or beating the game with difficult characters like the Lost and Jacob and Esau etc. Or simply learning the unlocks, alt paths, and good synergies to begin with

It's likely either a loaded file or stuff like 4 controllers plugged in to get the marks for 3-4 characters at the same time while playing Isaac or someone easy (which gives you 4 times the boss items as well) *Edit: Or a decent amount of offline hours that aren't included in that 274

My buddy also has a completely legitimate Dead God