He artificially constrained refterm to use DirectText specifically, because that's what the Windows Terminal team was using, and Casey wanted to prove that this

He artificially constrained refterm to use DirectText specifically, because that's what the Windows Terminal team was using, and Casey wanted to prove that this could be made performant. The specific argument the Microsoft guys were making was that DirectText/Direct2D had inherent limitations that fundamentally prevented certain scenarios running fast, such as a mix of many different colours. Casey proved them wrong, but to do that he had to use the same underlying libraries so as not to be seen as somehow "cheating".