The cables are compatible, but… that’s not how it works. Where did you get that information from?

A Thunderbolt 3 cable can transfer up to 40 Gbps, twice the maximum of USB-C. However, these speeds are only possible when using a Thunderbolt cable with a Thunderbolt port, not a USB-C port. This is because Thunderbolt 3 uses two lanes of PCIe 3.0 for data transfer, while USB-C uses one lane of PCIe 3.0 or two lanes of PCIe 2.0.