Thunderbolt 3 allows 40 Gbps in each direction using two 20 Gbps channels for each direction. As a protocol driving a display is mainly unidirectional, DisplayPort 2.0 simply uses all four channel for transmitting the data to the display, resulting in 80 Gbps. So they're not squeezing more data through the connector and the cables than Thunderbolt 3 already does, but simply do it in one direction only.

Ah, that makes sense. There's probably still room in the copper for more than 20 Gbps per lane, if they switch to an encoding with more than one voltage level, like PAM4.

The USB-C connector physically supports 20 Gbit/s per pin in the USB4 spec (4 pins total: 2 rx + 2 tx, that's why we say it's "40 Gbit/s" per direction). DisplayPort configures all 4 pins as tx, so as expected it achieves 4 x 20 = 80 Gbit/s.