Migration Assistant: Thunderbolt to USB-C slow?

I'm trying to do a migration from my old MacBook Pro Retina (Late 2013) to my new MacBook Pro (Late 2016). I have connected a Thunderbolt cable on the old MacBook to a USB-C Thunderbolt adapter on the new MacBook. The old MacBook is set to Target Disk Mode. I've confirmed that the dialog said it would use the Thunderbolt connection. After 8 hours of migration it is still going to take 2 hours at a whopping speed of 8 MB/s. As I want to get my money's worth of that ThunderBolt cable, I've cancelled the migration. I've read some reports about Target Disk Mode being slow, so I've restarted my old MacBook and am trying to get Migration Assistant to run without TDM.

However the old MacBook shows up, it gives an error about not being able to connect. My suspicion is that it ignores the Thunderbolt connection and tries peer-to-peer WiFi instead. According to Move content to your new MacBook or late-2016 MacBook Pro you can only use Migration Assistant with USB-C in Target Disk Mode. If found to be true, would make me very sad. Seeing that both MacBooks are equipped with SSD, 8 MB/s is joke.