Normally, in Ethernet World, bit rate is referring to both downlink and uplink separately. So, terabit Ethernet is likely referring to 1Tb up and 1Tb down. However, the PHY and LINK layer support 1Tb doesn't always mean a device can dump so much data into the link.
5nm is probably referred to physical limitation which is bounded by law of physics. I would love to hear from material or semi conductor professional to confirm.
Finally someone speaks out honestly about this.
Broadcom CTO Henry Samueli at #ethernet40 celebration:
Moore’s Law is coming to an end.
Transistors will stop scaling around 5 nm and everything will plateau.
I am comfortable we will get to terabit speeds but I’m not sure we will see petabit.
You will see density of switch boards level off. It will change the dynamics of the entire industry.
We still have another 15 years or so to enjoy but we need to prepare at some point for a network that doesn’t double bandwidth every two years."
What do you think about them bones?!?