"SIM cards as private keys?" This doesn't seem a good innovation where security is of utmost consideration.
No, your SIM card isn't used as private key. Phone number of that SIM card is used to login on their centralized service. But obviously it has different security concern.
Yeah, I get it. I was just reacting to what was written on the article. Those were the exact words, that the developer was innovating "for a way to use SIM cards as private keys."
In a way, it could be likened to it. Having access to the SIM card means having access to the phone number which is like the address, where the load or points or credit is sent. Although if a PIN is needed not just when you register an account but every time you will send or receive Bitcoin, the SIM isn't entirely comparable to private keys. But, just the same, this is essentially more or less like how prepaid SMS load works. It's just that it is Bitcoin-denominated and not fiat. But, of course, you don't really have real Bitcoin in this case but just credit or load redeemable for Bitcoin.