Inside "decentralization" there are two distinct concepts that are constantly confused, you see and hear this often: diffusion (or being "distributed") and control.
The ideal centralized sequencer is clearly not diffuse, but its operator lacks control. If all it can do is process transactions properly or stop running, we can trust it ("credible neutrality"). This is achieved without diffusion at all.
"Decentralization" in this sense is a means to an end: a system that lacks a single controller that you need to trust.