It expands their empire like Microsoft pledging to "support" Open Source: it's disingenuous, self-serving, and develops a "claim" of authority over the sector. It allows them, the makers of Unreal Engine, to develop a business relationship with their competition and influence the trajectory on one of only major alternatives in order to control the market more.
If Epic Games really cared about Godot, they would align more with their values in-house. Their M&A drives the organization like a propeller.
It's all hypothetical for a transaction 5 years in the past. The future you propose is one where Epic is not actually the same: they have more liquid capital towards the mission their stakeholders decide, and less influence on Godot.
However, their stakeholders decided circa 2019/2020 that they want to influence the development of Godot and spent their money that way. Corporate donations aren't at a whim like us individuals who spend $3/mo on Wikipedia or a food pantry, it's considered by the executive team, calculated and green-lit by their accounting team.
If Epic Games really cared about Godot, they would align more with their values in-house. Their M&A drives the organization like a propeller.