When two devices connect via wifi, is the connection 'direct' or are they going through a wifi network? In the documentation for 'How to move your content to a new Mac', we see:

If both computers are using macOS Sierra or later, just make sure that they're near each other and have Wi-Fi turned on. If either is using OS X El Capitan or earlier, connect them to the same network using Wi-Fi or Ethernet.

I presume it's implicit that if both computers are using macOS Sierra or later, they'll connect directly to each other (without requiring a separate wifi network)? Is this correct?
The reason I ask is because I currently have access to only slow wifi networks, which would pose a problem with the amount of data needing transfer.
It doesn’t matter if you have a router or not, the macs will find each other and transfer. Plug a direct ethernet cable if you have it for fastest speeds, however. Your reading is correct - good work picking apart the nuance of direct connection on Sierra.