Are you suggesting the primary reason SPJ moved to Epic Games is because they offered him more money than Microsoft?
Reading the message from SPJ[0] seems to indicate he is excited for the people and projects he will be working on as well as being given the freedom to continue working on education, functional programming research, and continuing to work in the Haskell ecosystem.
Of course large amounts of money are involved for an engineer of SPJ's renown and experience - that requirement exists for any company that want's SPJ's time.
[0] https://discourse.haskell.org/t/an-epic-future-for-spj/3573