>> To achieve its impressive data transfer rates, PCIe 7.0 doubles the bus frequency at the physical layer compared to PCIe 5.0 and 6.0. Otherwise, the standard retains pulse amplitude modulation with four level signaling (PAM4), 1b/1b FLIT mode encoding, and the forward error correction (FEC) technologies that are already used for PCIe 6.0.

Nothing else changed, they didn't move to a different encoding scheme. PCIe 6.0 already uses PAM4. Unless they moved to a higher density PAM scheme (which they didn't), the only way to increase bandwidth is to increase speed.