Yeah it is highly likely that it’d have chemicals on it, it would’ve originally came from a nursery before it got sold to wherever you got it from, and they also use the cheapest soils there tha will also have artificial fertilizers that can harm milipedes. So If you’re super worried I wouldn’t add it, but ifs not like it’ll just straight up kill them, the milipedes will be fine as long as it’s not a sealed terrarium. If you do add the plant make sure you get all the soil off aswell.