Yes the use of organic matter to backfill planting holes is one of the myths I mention, its definitely best with soil only, then mulch on top.
You need to encourage soil life to improve structure and drainage so any mulch you can get on after planting is good, even if only a light dressing of some compost, some fresh material, and then a fuller mulch in March is a good plan.
I hope the rain holds off for your plantings.
