It is not currently possible to offer backordering to only wholesale customers, nor is it possible to offer backordering to only retail customers.
Unfortunately, this isn't currently possible through the app, due to the fact that both your retail and wholesale customers are sharing a single Shopify listing. That being said, we do have a workaround available via our Product Exclusions setting which could be of some help with this. You can learn more about this suggested fix here.