Unlike previous consumer "micro" and "pico" cells, the Magic Box connects to a nearby Sprint cell tower and does not your use home's internet connection to connect to Sprint's network. This means it is not intended to provide cell coverage in areas with no or marginal signal. Instead, it's intended to speed up Sprint's "Network Densification Strategy".
The upcoming move to 5G mobile networks, which is still years away, will require smaller cells to deliver the promised higher speeds. But those cells are also likely to require wired backhaul, not wireless, not to mention different radios. So the Magic Box really has nothing to do with 5G.
Sprint says it has wireless backhaul capacity in the form of 204 MHz of spectrum, 160 MHz of which is in the 2.5 GHz band. The announcement also cited four-channel carrier aggregation, 256 QAM, 4x4 MIMO (multiple-input, multiple-output) and Massive MIMO technologies as being key to providing a speedy network without relying on wired backhaul or Wi-Fi.
Surprisingly, LTE-U was not cited as one of the "advanced technologies" used by Magic Box, which is puzzling given T-Mobile and AT&T's plans to use that technology to expand network capacity. Perhaps Sprint wants to avoid the controversy that still surrounds LTE-U, given its use of the same 5 GHz band used by Wi-Fi.
Sprint's consumer Magic Box page offers no hard speed / throughput specs, citing only 5 bars of signal, "average" 200% increase in up/download speeds and approximately 30,000 sq. ft. of indoor coverage. The page does say Magic Box "doesn't interfere with Wi-Fi" and best of all is offered at no cost to consumers. The press release contains more information, but again, no throughput specs.
If you're interested in hosting part of Sprint's network in your very own home, you can hit the Magic Box page linked above to sign up. But expect Sprint to be pretty picky and you'd better have a 5 bar signal already at your home.