the past drivers of margin expansion are largely baked into current results
Have any more context? It's not clear (to me, anyway) exactly what is being called "past drivers of margin expansion."
I assume it's something like this: Say you want to bake a pie. If you want an apple pie, you put apples in before you bake it. Cherries if you want cherry pie.
Once the pie comes out, you can't change it now. The ingredients are "baked in."