When tunneling, you are limited to DP 1.4a and wouldn't be able to do 8k/60Hz without compression (which is also optional, I believe). In alternate mode, which essentially runs DP over the USB cable without wrapping the protocol, you can use DP 2.0 and should be able to do 8k/60Hz. 8k/60Hz requires more than 25.92 Gbit/s (which is what you get in DP 1.4 HBR3, 32.40 Gbit/s including the encoding symbols), so I assume the alternate mode is not actually limited to the 40 Gbit/s bandwidth of USB4. Because it is unidirectional, I believe it can use up to 80 GBit/s.
It looks like Thunderbolt 3 only supports DP 1.4 (optionally, only 1.2 is mandatory), and you'd be limited to the same speeds as tunneling in USB4.
Honestly, if we are confused about this then I have no idea how the general public can be expected to know what their ports are capable of.