The mini is good (I've got one) but they don't use it for billing data.
It registers as an IHD and sends the data to them over your broadband, this is then used to show the live data in the app but also fills in the current day consumption. Overnight that data is replaced with the authoritative data via DCC.
The reason this is not done is that there is obviously a great incentive to tamper with the billing data given the cost of energy. It is extremely difficult to intercept GSM/LTE or indeed Longwave airside connections. With the Comms hardware in the meter it's also near impossible to physically tinker with it - at least not in a way that doesn't leave evidence that you've done so
However routing through a network the home owner has full control over opens up a lot more possiblity of interception, even good encryption can be vulnerable if you have control over the hardware and have time to analyse. It only needs enough smart people to figure it out and publish a project on GitHub or sell a magic box on eBay.
So while I can see more folks doing clever things with making the data available for monitoring and automation (and the standards expected that, it's why you have devices that can join the local meter ZigBee network like an IHD) I think it will be a good long while before the authoritative data used for billing goes through homeowners networks.