How does CalDigit Thunderbolt 3 Mini Dock support two displays from one USB-C port?
The clue is in the name: Thunderbolt and USB are two different things, even though they both use the same plug and socket. 

USB C offers some of the capabilities - video signal, power transmission, data; but Thunderbolt provides a greater range of data travelling down the same cable.

The Thunderbolt Dock has 2 HDMI ports, which can provide two separate video outputs. The USB hub has one HDMI port.