Assuming the Withings app is on the same phone as the HealthKit account, there's no "cloud" involved. This is 100% grift. I hope nobody buys these.
Required path is: Scale data -> WiFi -> Internet -> Withings app -> HealthKit
Well, fuck that. Now I'm really, really, really not going to buy a Withings product again.
Never again.
In what room did they possibly imagine their device would be used?
Looks like HA updated their documentation to reflect the $99/yr requirement: https://www.home-assistant.io/integrations/withings/