Wi-Fi Calling is a Sprint-specific function. It does not affect TextNow wifi calling at all - in fact, if you try to turn it on you'll probably get an Enablement Error and the phone will keep the setting greyed out.
This particular setting can only be used by Sprint plan customers. As our TextNow system uses an entirely different system to place and receive calls on wifi, there's no need to turn it on. Any error messages you get with regards to this feature can be safely ignored.