Hello dwatterson,
We see you already reviewed the following resource, which provides the best information about this adapter for data transfer: About the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter - Apple Support:
"Data transfer
Thunderbolt or Thunderbolt 2 - You can also use the Thunderbolt port of this adapter to connect an external hard drive, dock, Mac, or other device that uses a Thunderbolt or Thunderbolt 2 cable for data transfer. This adapter supports data-transfer speeds up to 10Gbps with Thunderbolt devices, and up to 20Gbps with Thunderbolt 2 devices.
This adapter is bidirectional, which means you can use it to connect Thunderbolt 3 devices to a Mac that has a Thunderbolt or Thunderbolt 2 port. In that case, the Mac must be using macOS Sierra or later, and the device using Thunderbolt 3 must provide its own power."
We hope this helps. Let us know if we can assist with anything else at all. Take care.