2-4 inch gap between crawlspace slab (floor) and surrounding wall - Why?
Almost certainly for drainage. Carrying any water leaking in from the walls to the sump.

My understanding of radon mitigation would be that this gap and the sump pit need to be sealed from the top and power vented to the exterior. Any underslab drainage on the "basement" side should be connected to the vent system as well.

If you already had a radon mitigation system in place and the company did work this way they are a bit clueless/irresponsible. If you did not have such a system, you evidently need one now.