There's a few things in 5G that make it more friendly towards massive numbers of small, low-bandwidth devices.
1. Unlicensed spectrum usage. So anyone can make a device that does short-range communication amongst sensors or with a hub.
2. Low-bandwidth, single-mode LTE. Arguably already existent in 4G but no real unified standard so far.
It's possible to do sea-of-sensors type devices today with chips from various vendors, but they're all geared towards closed and proprietary networks of machines deployed all from the same vendor/ISP that uses that ISP's licensed spectrum.
5G lets small, low-power cellular devices hop on unlicensed spectrum, which lets any vendor deploy it.