Windows TechPreview can’t be installed to Thinkpad8 now.
The issue is Wifi driver. Wifi chip on Thinkpad 8 is BCM4324. Windows recognized the device but cannot start the device (code 10 error).
Without wifi connection, it’s possible to evaluate Windows Techpreview on thinkpad 8. I believe that there is no meaning to use Windows Techpreview without wireless connection.