Re: "...only from within Sprint's network."
No, I'm not reassured, but the article is inaccurate. No, it's not a relief, but there is a measure of segregation that would prevent any random hacker from easily gaining access. I haven't seen this mentioned anywhere, but I know for other Sprint implementations over their CDMA and LTE networks, they use it as an extension of the MPLS service, so a general PAYG 3G/4G modem wouldn't even slightly help without a lot of social engineering to get the right authorizations in place.
Biting the hand that feeds IT
